diff --git a/components/admin/tabs/storage.js b/components/admin/tabs/storage.js
index 30ca3412..eadd487f 100644
--- a/components/admin/tabs/storage.js
+++ b/components/admin/tabs/storage.js
@@ -186,7 +186,7 @@ export default function StorageTab({ data, repCompleteCount }) {
                                                         onClick={async () => {
                                                             const response =
                                                                 await fetch(
-                                                                    "/api/file/editRepJson",
+                                                                    "/api/admin/editRepJson",
                                                                     {
                                                                         method: "POST",
                                                                         headers:
diff --git a/docs/administrative-authentication.md b/docs/administrative-authentication.md
index bc212df9..44a941c0 100644
--- a/docs/administrative-authentication.md
+++ b/docs/administrative-authentication.md
@@ -70,16 +70,14 @@ independent administrative-session authorization. A later authorised stage
 must enforce the administrative session on privileged API routes and account
 for both browser calls and the existing server-side bootstrap callers.
 
-Storage download, representation editing, and queue actions retain their
-existing authorization behaviour. The other administrative API and storage
-limitations described in this section remain open.
+Storage download and queue actions retain their existing authorization
+behaviour. The other administrative API and storage limitations described in
+this section remain open.
 
 Directly observed storage routes include:
 
 - `/api/file/getrepsblobproxy`, which forwards a caller-supplied container using
   server relay credentials and path signing.
-- `/api/file/editRepJson`, which performs a direct blob mutation and has no
-  visible administrative-session or HMAC check.
 - `/api/file/downloadblob` and `/api/file/createrepcompletemessage_api`, which
   enforce existing path-HMAC checks but not administrative authorization.
 
@@ -97,6 +95,12 @@ the administrative session and requires a trusted same-origin request. It also
 validates the requested container, case-folder segment, and representation-file
 segment before performing deletion.
 
+Administrative representation JSON unlocking uses the dedicated
+`POST /api/admin/editRepJson` route. That endpoint independently validates the
+administrative session, requires a trusted same-origin request, and validates
+the requested container and blob path before removing `repComplete`. The former
+`/api/file/editRepJson` route is retired and no longer performs storage access.
+
 Administrative case deletion was not investigated or remediated as part of
 Stage 3 and remains a separate, unverified security boundary. The other
 administrative API and storage limitations documented above also remain open.
diff --git a/pages/api/admin/editRepJson.js b/pages/api/admin/editRepJson.js
new file mode 100644
index 00000000..4306cd81
--- /dev/null
+++ b/pages/api/admin/editRepJson.js
@@ -0,0 +1,126 @@
+import { DefaultAzureCredential } from "@azure/identity";
+import { BlobServiceClient } from "@azure/storage-blob";
+import { consoleLogger } from "../../../actions/core/logger";
+import {
+    hasTrustedSameOrigin,
+    readAdminSessionCookie,
+    setAdminAuthNoStore,
+    validateAdminSession
+} from "../../../lib/admin/adminAuth";
+import { respondError, respondSuccess } from "../middleware/apiResponse";
+
+const forbidden = (res) =>
+    respondError(res, {
+        status: 403,
+        code: "FORBIDDEN",
+        message: "Request not permitted"
+    });
+
+const isValidContainer = (value) =>
+    typeof value === "string" &&
+    value.length >= 3 &&
+    value.length <= 63 &&
+    /^[a-z0-9](?:[a-z0-9-]*[a-z0-9])?$/.test(value);
+
+const isValidBlobName = (value) =>
+    typeof value === "string" &&
+    value.length > 0 &&
+    value.length <= 1024 &&
+    value === value.trim() &&
+    !/[\\\u0000-\u001f\u007f]/.test(value) &&
+    value.split("/").every((segment) => segment && segment !== "." && segment !== "..");
+
+export default async function handler(req, res) {
+    setAdminAuthNoStore(res);
+    res.setHeader("Allow", "POST");
+
+    if (req.method !== "POST") {
+        return respondError(res, {
+            status: 405,
+            code: "METHOD_NOT_ALLOWED",
+            message: "Method not allowed"
+        });
+    }
+
+    const adminSession = readAdminSessionCookie(req);
+    if (
+        !validateAdminSession(adminSession) ||
+        !hasTrustedSameOrigin(req)
+    ) {
+        return forbidden(res);
+    }
+
+    const { container, blobName } = req.body ?? {};
+    if (!isValidContainer(container) || !isValidBlobName(blobName)) {
+        return respondError(res, {
+            status: 400,
+            code: "INVALID_REQUEST",
+            message: "Invalid request"
+        });
+    }
+
+    try {
+        const accountName = process.env.AZURE_STORAGE_ACCOUNT_NAME;
+        const creds = new DefaultAzureCredential();
+        const blobServiceClient = new BlobServiceClient(
+            `https://${accountName}.blob.core.windows.net`,
+            creds
+        );
+
+        const containerClient = blobServiceClient.getContainerClient(container);
+        const blockBlobClient = containerClient.getBlockBlobClient(blobName);
+
+        const downloadResponse = await blockBlobClient.download(0);
+        const downloaded = await streamToString(
+            downloadResponse.readableStreamBody
+        );
+        const json = JSON.parse(downloaded);
+
+        if ("repComplete" in json) {
+            delete json.repComplete;
+        }
+
+        const tags = {
+            containerid: json.containerID,
+            caseID: json.ticketnumber,
+            blobType: "Representation"
+        };
+
+        const updatedContent = JSON.stringify(json, null, 2);
+
+        await blockBlobClient.upload(
+            updatedContent,
+            Buffer.byteLength(updatedContent),
+            {
+                blobHTTPHeaders: { blobContentType: "application/json" }
+            }
+        );
+
+        await blockBlobClient.setTags(tags);
+        await blockBlobClient.setMetadata(tags);
+
+        return respondSuccess(res, {
+            message: "repComplete removed successfully"
+        });
+    } catch (error) {
+        consoleLogger(error);
+        return respondError(res, {
+            status: 500,
+            code: "EDIT_REP_JSON_FAILED",
+            message: "Failed to edit rep.json"
+        });
+    }
+}
+
+async function streamToString(readableStream) {
+    return new Promise((resolve, reject) => {
+        const chunks = [];
+        readableStream.on("data", (data) => {
+            chunks.push(data.toString());
+        });
+        readableStream.on("end", () => {
+            resolve(chunks.join(""));
+        });
+        readableStream.on("error", reject);
+    });
+}
\ No newline at end of file
diff --git a/pages/api/file/editRepJson.js b/pages/api/file/editRepJson.js
index e301ade4..ad7b3dd1 100644
--- a/pages/api/file/editRepJson.js
+++ b/pages/api/file/editRepJson.js
@@ -1,96 +1,9 @@
-import { DefaultAzureCredential } from "@azure/identity";
-import { BlobServiceClient } from "@azure/storage-blob";
-import { consoleLogger } from "../../../actions/core/logger";
-import { respondError, respondSuccess } from "../middleware/apiResponse";
+import { respondError } from "../middleware/apiResponse";
 
 export default async function handler(req, res) {
-    if (req.method !== "POST") {
-        return respondError(res, {
-            status: 405,
-            code: "METHOD_NOT_ALLOWED",
-            message: "Method not allowed"
-        });
-    }
-
-    const { container, blobName } = req.body;
-
-    if (!container || !blobName) {
-        return respondError(res, {
-            status: 400,
-            code: "MISSING_REQUIRED_BODY",
-            message: "Missing container or blobName"
-        });
-    }
-
-    try {
-        const accountName = process.env.AZURE_STORAGE_ACCOUNT_NAME;
-        const creds = new DefaultAzureCredential();
-        const blobServiceClient = new BlobServiceClient(
-            `https://${accountName}.blob.core.windows.net`,
-            creds
-        );
-
-        const containerClient = blobServiceClient.getContainerClient(container);
-        const blockBlobClient = containerClient.getBlockBlobClient(blobName);
-
-        // Download blob content
-        const downloadResponse = await blockBlobClient.download(0);
-        const downloaded = await streamToString(
-            downloadResponse.readableStreamBody
-        );
-
-        // Parse JSON
-        const json = JSON.parse(downloaded);
-
-        // Remove repComplete element if it exists
-        if ("repComplete" in json) {
-            delete json.repComplete;
-        }
-
-        const tags = {
-            containerid: json.containerID,
-            caseID: json.ticketnumber,
-            blobType: "Representation"
-        };
-
-        // Convert back to string
-        const updatedContent = JSON.stringify(json, null, 2);
-
-        // Upload updated content (overwrite)
-        await blockBlobClient.upload(
-            updatedContent,
-            Buffer.byteLength(updatedContent),
-            {
-                blobHTTPHeaders: { blobContentType: "application/json" }
-            }
-        );
-
-        const withTags = await blockBlobClient.setTags(tags);
-        const withMeta = await blockBlobClient.setMetadata(tags);
-
-        return respondSuccess(res, {
-            message: "repComplete removed successfully"
-        });
-    } catch (error) {
-        consoleLogger(error);
-        return respondError(res, {
-            status: 500,
-            code: "EDIT_REP_JSON_FAILED",
-            message: "Failed to edit rep.json"
-        });
-    }
-}
-
-// Helper function to read stream to string
-async function streamToString(readableStream) {
-    return new Promise((resolve, reject) => {
-        const chunks = [];
-        readableStream.on("data", (data) => {
-            chunks.push(data.toString());
-        });
-        readableStream.on("end", () => {
-            resolve(chunks.join(""));
-        });
-        readableStream.on("error", reject);
+    return respondError(res, {
+        status: 410,
+        code: "ENDPOINT_RETIRED",
+        message: "Endpoint retired"
     });
 }
diff --git a/tests/phase21/file-handler-contract.test.cjs b/tests/phase21/file-handler-contract.test.cjs
index 1eeab609..1d40d659 100644
--- a/tests/phase21/file-handler-contract.test.cjs
+++ b/tests/phase21/file-handler-contract.test.cjs
@@ -573,6 +573,243 @@ test("admin storage caller uses dedicated POST deletion while ordinary caller re
     );
 });
 
+const loadAdminRepJsonEdit = (overrides = {}) => {
+    let azureClientConstructions = 0;
+    const storageCalls = [];
+    const sourceJson = {
+        containerID: "ck-user-container",
+        ticketnumber: "CASE-1",
+        repComplete: true,
+        representationType: "Support",
+        nested: { retained: true }
+    };
+    const uploaded = {};
+
+    class DefaultAzureCredentialMock {}
+
+    class BlobServiceClientMock {
+        constructor(url, credential) {
+            azureClientConstructions += 1;
+            storageCalls.push(["construct", url, credential]);
+        }
+
+        getContainerClient(container) {
+            storageCalls.push(["container", container]);
+            return {
+                getBlockBlobClient(blobName) {
+                    storageCalls.push(["blob", blobName]);
+                    return {
+                        async download(offset) {
+                            storageCalls.push(["download", offset]);
+                            const { Readable } = require("stream");
+                            return {
+                                readableStreamBody: Readable.from([
+                                    JSON.stringify(sourceJson)
+                                ])
+                            };
+                        },
+                        async upload(content, length, options) {
+                            uploaded.content = content;
+                            uploaded.length = length;
+                            uploaded.options = options;
+                            storageCalls.push(["upload"]);
+                        },
+                        async setTags(tags) {
+                            uploaded.tags = tags;
+                            storageCalls.push(["tags"]);
+                        },
+                        async setMetadata(metadata) {
+                            uploaded.metadata = metadata;
+                            storageCalls.push(["metadata"]);
+                        }
+                    };
+                }
+            };
+        }
+    }
+
+    const mod = loadModule("pages/api/admin/editRepJson.js", {
+        Buffer,
+        DefaultAzureCredential: DefaultAzureCredentialMock,
+        BlobServiceClient: BlobServiceClientMock,
+        consoleLogger: () => {},
+        readAdminSessionCookie: () => "admin-token",
+        validateAdminSession: () => true,
+        hasTrustedSameOrigin: () => true,
+        setAdminAuthNoStore: () => {},
+        respondError: respondErrorMock,
+        respondSuccess: respondSuccessMock,
+        ...overrides
+    });
+
+    return {
+        mod,
+        getAzureClientConstructions: () => azureClientConstructions,
+        storageCalls,
+        sourceJson,
+        uploaded
+    };
+};
+
+const validAdminRepJsonEditRequest = () => ({
+    method: "POST",
+    headers: { origin: "https://portal.example.test" },
+    body: {
+        container: "ck-user-container",
+        blobName: "CASE-1/representation-1/representation_rep.json"
+    }
+});
+
+test("admin representation JSON edit forbids a missing administrative session before Azure access", async () => {
+    const loaded = loadAdminRepJsonEdit({
+        readAdminSessionCookie: () => null,
+        validateAdminSession: () => false
+    });
+    const res = createRes();
+
+    await loaded.mod.default(validAdminRepJsonEditRequest(), res);
+
+    assert.strictEqual(res.state.statusCode, 403);
+    assert.strictEqual(res.state.jsonBody.error.code, "FORBIDDEN");
+    assert.strictEqual(loaded.getAzureClientConstructions(), 0);
+});
+
+test("admin representation JSON edit forbids an invalid administrative session before Azure access", async () => {
+    const loaded = loadAdminRepJsonEdit({ validateAdminSession: () => false });
+    const res = createRes();
+
+    await loaded.mod.default(validAdminRepJsonEditRequest(), res);
+
+    assert.strictEqual(res.state.statusCode, 403);
+    assert.strictEqual(loaded.getAzureClientConstructions(), 0);
+});
+
+test("admin representation JSON edit forbids cross-origin requests before Azure access", async () => {
+    const loaded = loadAdminRepJsonEdit({ hasTrustedSameOrigin: () => false });
+    const req = validAdminRepJsonEditRequest();
+    req.headers.origin = "https://attacker.example";
+    const res = createRes();
+
+    await loaded.mod.default(req, res);
+
+    assert.strictEqual(res.state.statusCode, 403);
+    assert.strictEqual(loaded.getAzureClientConstructions(), 0);
+});
+
+test("admin representation JSON edit rejects unsupported methods before Azure access", async () => {
+    const loaded = loadAdminRepJsonEdit();
+    const req = validAdminRepJsonEditRequest();
+    req.method = "GET";
+    const res = createRes();
+
+    await loaded.mod.default(req, res);
+
+    assert.strictEqual(res.state.statusCode, 405);
+    assert.strictEqual(res.state.jsonBody.error.code, "METHOD_NOT_ALLOWED");
+    assert.strictEqual(res.state.headers.allow, "POST");
+    assert.strictEqual(loaded.getAzureClientConstructions(), 0);
+});
+
+test("admin representation JSON edit rejects malformed identifiers before Azure access", async () => {
+    for (const malformedBody of [
+        { container: "UPPERCASE" },
+        { container: "ab" },
+        { blobName: "../representation_rep.json" },
+        { blobName: "CASE-1//representation_rep.json" },
+        { blobName: "CASE-1\\representation_rep.json" },
+        { blobName: " representation_rep.json" }
+    ]) {
+        const loaded = loadAdminRepJsonEdit();
+        const req = validAdminRepJsonEditRequest();
+        req.body = { ...req.body, ...malformedBody };
+        const res = createRes();
+
+        await loaded.mod.default(req, res);
+
+        assert.strictEqual(res.state.statusCode, 400);
+        assert.strictEqual(res.state.jsonBody.error.code, "INVALID_REQUEST");
+        assert.strictEqual(loaded.getAzureClientConstructions(), 0);
+    }
+});
+
+test("valid administrative representation JSON edit preserves the existing mutation contract", async () => {
+    const loaded = loadAdminRepJsonEdit();
+    const res = createRes();
+
+    await loaded.mod.default(validAdminRepJsonEditRequest(), res);
+
+    assert.strictEqual(res.state.statusCode, 200);
+    assert.deepStrictEqual(JSON.parse(JSON.stringify(res.state.jsonBody)), {
+        message: "repComplete removed successfully"
+    });
+    assert.strictEqual(loaded.getAzureClientConstructions(), 1);
+    assert.deepStrictEqual(loaded.storageCalls.slice(1, 4), [
+        ["container", "ck-user-container"],
+        ["blob", "CASE-1/representation-1/representation_rep.json"],
+        ["download", 0]
+    ]);
+
+    const updatedJson = JSON.parse(loaded.uploaded.content);
+    assert.strictEqual(Object.hasOwn(updatedJson, "repComplete"), false);
+    assert.strictEqual(updatedJson.containerID, loaded.sourceJson.containerID);
+    assert.strictEqual(updatedJson.ticketnumber, loaded.sourceJson.ticketnumber);
+    assert.strictEqual(
+        updatedJson.representationType,
+        loaded.sourceJson.representationType
+    );
+    assert.deepStrictEqual(
+        JSON.parse(JSON.stringify(updatedJson.nested)),
+        loaded.sourceJson.nested
+    );
+    assert.strictEqual(
+        loaded.uploaded.length,
+        Buffer.byteLength(loaded.uploaded.content)
+    );
+    assert.deepStrictEqual(
+        JSON.parse(JSON.stringify(loaded.uploaded.options)),
+        { blobHTTPHeaders: { blobContentType: "application/json" } }
+    );
+    assert.deepStrictEqual(JSON.parse(JSON.stringify(loaded.uploaded.tags)), {
+        containerid: "ck-user-container",
+        caseID: "CASE-1",
+        blobType: "Representation"
+    });
+    assert.deepStrictEqual(
+        JSON.parse(JSON.stringify(loaded.uploaded.metadata)),
+        JSON.parse(JSON.stringify(loaded.uploaded.tags))
+    );
+});
+
+test("retired file representation JSON endpoint cannot mutate storage", async () => {
+    let storageCalls = 0;
+    const mod = loadModule("pages/api/file/editRepJson.js", {
+        BlobServiceClient: class {
+            constructor() {
+                storageCalls += 1;
+            }
+        },
+        respondError: respondErrorMock
+    });
+    const res = createRes();
+
+    await mod.default(validAdminRepJsonEditRequest(), res);
+
+    assert.strictEqual(res.state.statusCode, 410);
+    assert.strictEqual(res.state.jsonBody.error.code, "ENDPOINT_RETIRED");
+    assert.strictEqual(storageCalls, 0);
+});
+
+test("admin storage caller uses the protected representation JSON edit endpoint", () => {
+    const rootDir = path.resolve(__dirname, "..", "..");
+    const adminSource = fs.readFileSync(
+        path.join(rootDir, "components/admin/tabs/storage.js"),
+        "utf8"
+    );
+
+    assert.match(adminSource, /fetch\(\s*"\/api\/admin\/editRepJson"/);
+    assert.doesNotMatch(adminSource, /fetch\(\s*"\/api\/file\/editRepJson"/);
+});
+
 test("deleteawaitingsubmissionfromblob accepts legacy deleteblob hash variant", async () => {
     let deletedPath = null;
     const mod = loadModule(