diff --git a/packages/next/src/server/image-optimizer.ts b/packages/next/src/server/image-optimizer.ts
index 1a97536b9a6a0..87823065b6c8f 100644
--- a/packages/next/src/server/image-optimizer.ts
+++ b/packages/next/src/server/image-optimizer.ts
@@ -198,6 +198,12 @@ export class ImageOptimizerCache {
       return { errorMessage: '"url" parameter is too long' }
     }
 
+    if (url.startsWith('//')) {
+      return {
+        errorMessage: '"url" parameter cannot be a protocol-relative URL (//)',
+      }
+    }
+
     let isAbsolute: boolean
 
     if (url.startsWith('/')) {
diff --git a/test/integration/image-optimizer/app/pages/api/no-header.js b/test/integration/image-optimizer/app/pages/api/no-header.js
new file mode 100644
index 0000000000000..287931027217f
--- /dev/null
+++ b/test/integration/image-optimizer/app/pages/api/no-header.js
@@ -0,0 +1,5 @@
+export default function handler(_req, res) {
+  // Intentionally missing Content-Type header so that
+  // the fallback is not served when optimization fails
+  res.end('foo')
+}
diff --git a/test/integration/image-optimizer/test/util.ts b/test/integration/image-optimizer/test/util.ts
index 015e04363f162..83344cc4fbc12 100644
--- a/test/integration/image-optimizer/test/util.ts
+++ b/test/integration/image-optimizer/test/util.ts
@@ -987,8 +987,17 @@ export function runTests(ctx: RunTestsCtx) {
     expect(await res.text()).toBe(`"url" parameter is too long`)
   })
 
+  it('should fail when url is protocol relative', async () => {
+    const query = { url: `//example.com`, w: ctx.w, q: 1 }
+    const res = await fetchViaHTTP(ctx.appPort, '/_next/image', query, {})
+    expect(res.status).toBe(400)
+    expect(await res.text()).toBe(
+      `"url" parameter cannot be a protocol-relative URL (//)`
+    )
+  })
+
   it('should fail when internal url is not an image', async () => {
-    const url = `//<h1>not-an-image</h1>`
+    const url = `/api/no-header`
     const query = { url, w: ctx.w, q: 39 }
     const opts = { headers: { accept: 'image/webp' } }
     const res = await fetchViaHTTP(ctx.appPort, '/_next/image', query, opts)