diff --git a/readthedocs/projects/tasks/builds.py b/readthedocs/projects/tasks/builds.py
index 41d79b11c9d..9e409f91263 100644
--- a/readthedocs/projects/tasks/builds.py
+++ b/readthedocs/projects/tasks/builds.py
@@ -574,24 +574,13 @@ def get_valid_artifact_types(self):
 
             if artifact_type == "html":
                 index_html_filepath = os.path.join(artifact_directory, "index.html")
-                readme_html_filepath = os.path.join(artifact_directory, "README.html")
-                if not os.path.exists(index_html_filepath) and not os.path.exists(
-                    readme_html_filepath
-                ):
-                    log.warning(
+                if not os.path.exists(index_html_filepath):
+                    log.info(
                         "Failing the build. "
                         "HTML output does not contain an 'index.html' at its root directory.",
                         index_html=index_html_filepath,
-                        readme_html=readme_html_filepath,
                     )
-                    # TODO: uncomment this line to fail the build once we have
-                    # communicated with projects without an index.html or
-                    # README.html
-                    #
-                    # NOTE: we want to deprecate serving README.html as an
-                    # index.html file as well.
-                    #
-                    # raise BuildUserError(BuildUserError.BUILD_OUTPUT_HTML_NO_INDEX_FILE)
+                    raise BuildUserError(BuildUserError.BUILD_OUTPUT_HTML_NO_INDEX_FILE)
 
             if not os.path.exists(artifact_directory):
                 # There is no output directory.
diff --git a/readthedocs/projects/tests/test_build_tasks.py b/readthedocs/projects/tests/test_build_tasks.py
index a69f9c19b6c..547da8c42e3 100644
--- a/readthedocs/projects/tests/test_build_tasks.py
+++ b/readthedocs/projects/tests/test_build_tasks.py
@@ -294,6 +294,14 @@ def test_build_updates_documentation_type(self, load_yaml_config):
                 )
             ).touch()
 
+        # Create an "index.html" at root to avoid failing the builds
+        pathlib.Path(
+            os.path.join(
+                self.project.artifact_path(version=self.version.slug, type_="html"),
+                "index.html",
+            )
+        ).touch()
+
         self._trigger_update_docs_task()
 
         # Update version state
@@ -438,6 +446,14 @@ def test_successful_build(
                 )
             ).touch()
 
+        # Create an "index.html" at root to avoid failing the builds
+        pathlib.Path(
+            os.path.join(
+                self.project.artifact_path(version=self.version.slug, type_="html"),
+                "index.html",
+            )
+        ).touch()
+
         self._trigger_update_docs_task()
 
         # It has to be called twice, ``before_start`` and ``after_return``