diff --git a/lib/util/merge-confidence/index.spec.ts b/lib/util/merge-confidence/index.spec.ts
index 6c9e379abe23cc..9b672ba0ef78af 100644
--- a/lib/util/merge-confidence/index.spec.ts
+++ b/lib/util/merge-confidence/index.spec.ts
@@ -347,6 +347,23 @@ describe('util/merge-confidence/index', () => {
         );
       });
 
+      it('uses a custom base url containing path', async () => {
+        const renovateApi = 'https://domain.com/proxy/renovate-api';
+        process.env.RENOVATE_X_MERGE_CONFIDENCE_API_BASE_URL = renovateApi;
+        httpMock.scope(renovateApi).get(`/api/mc/availability`).reply(200);
+
+        await expect(initMergeConfidence()).toResolve();
+
+        expect(logger.trace).toHaveBeenCalledWith(
+          expect.anything(),
+          'using merge confidence API base found in environment variables',
+        );
+        expect(logger.debug).toHaveBeenCalledWith(
+          expect.anything(),
+          'merge confidence API - successfully authenticated',
+        );
+      });
+
       it('resolves if no token', async () => {
         hostRules.clear();
 
diff --git a/lib/util/merge-confidence/index.ts b/lib/util/merge-confidence/index.ts
index 95386836a04a36..56f2f177a52870 100644
--- a/lib/util/merge-confidence/index.ts
+++ b/lib/util/merge-confidence/index.ts
@@ -7,6 +7,7 @@ import * as packageCache from '../cache/package';
 import { parseJson } from '../common';
 import * as hostRules from '../host-rules';
 import { Http } from '../http';
+import { ensureTrailingSlash, joinUrlParts } from '../url';
 import { MERGE_CONFIDENCE } from './common';
 import type { MergeConfidence } from './types';
 
@@ -164,7 +165,14 @@ async function queryApi(
   }
 
   const escapedPackageName = packageName.replace('/', '%2f');
-  const url = `${apiBaseUrl}api/mc/json/${datasource}/${escapedPackageName}/${currentVersion}/${newVersion}`;
+  const url = joinUrlParts(
+    apiBaseUrl,
+    'api/mc/json',
+    datasource,
+    escapedPackageName,
+    currentVersion,
+    newVersion,
+  );
   const cacheKey = `${token}:${url}`;
   const cachedResult = await packageCache.get(hostType, cacheKey);
 
@@ -217,7 +225,7 @@ export async function initMergeConfidence(): Promise<void> {
     return;
   }
 
-  const url = `${apiBaseUrl}api/mc/availability`;
+  const url = joinUrlParts(apiBaseUrl, 'api/mc/availability');
   try {
     await http.get(url);
   } catch (err) {
@@ -246,7 +254,7 @@ function getApiBaseUrl(): string {
       { baseUrl: parsedBaseUrl },
       'using merge confidence API base found in environment variables',
     );
-    return parsedBaseUrl;
+    return ensureTrailingSlash(parsedBaseUrl);
   } catch (err) {
     logger.warn(
       { err, baseFromEnv },