diff --git a/docs/iris/src/Makefile b/docs/iris/src/Makefile
index 0aa921fd2a..2e47057a30 100644
--- a/docs/iris/src/Makefile
+++ b/docs/iris/src/Makefile
@@ -2,12 +2,15 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    = 
+SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = _build
 SRCDIR        = .
 
+# See https://www.sphinx-doc.org/en/master/man/sphinx-build.html?highlight=--keep-going#cmdoption-sphinx-build-W
+WARNING_TO_ERROR = -W --keep-going
+
 # Internal variables.
 PAPEROPT_a4     = -D latex_paper_size=a4
 PAPEROPT_letter = -D latex_paper_size=letter
@@ -39,7 +42,7 @@ clean:
 	-rm -rf $(SRCDIR)/generated
 
 html:
-	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	$(SPHINXBUILD) $(WARNING_TO_ERROR) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html"
 
diff --git a/docs/iris/src/developers_guide/contributing_documentation.rst b/docs/iris/src/developers_guide/contributing_documentation.rst
index b64d04875c..9674289568 100644
--- a/docs/iris/src/developers_guide/contributing_documentation.rst
+++ b/docs/iris/src/developers_guide/contributing_documentation.rst
@@ -46,11 +46,9 @@ If you wish to run a clean build you can run::
 
 This is useful for a final test before committing your changes.
 
-.. note:: In addition to the automated `travis-ci`_  build of the documentation,
-          the https://readthedocs.org/ service is also used.  The configuration
-          of this held in a file in the root of the
-          `github Iris project <https://github.com/SciTools/iris>`_  named
-          ``.readthedocs.yml``.
+.. note:: In order to preserve a clean build for the html, all **warnings**
+          have been promoted to be **errors** to ensure they are addressed.
+          This **only** applies when ``make html`` is run.
 
 .. _travis-ci: https://travis-ci.org/github/SciTools/iris
 
@@ -101,8 +99,13 @@ or ignore the url.
     ``spelling_word_list_filename``.
 
 
-.. note:: All of the above tests are automatically run as part of the
-          `travis-ci`_  automated build.
+.. note:: In addition to the automated `travis-ci`_ build of all the
+          documentation build options above, the
+          https://readthedocs.org/ service is also used.  The configuration
+          of this held in a file in the root of the
+          `github Iris project <https://github.com/SciTools/iris>`_  named
+          ``.readthedocs.yml``.
+
 
 .. _conf.py: https://github.com/SciTools/iris/blob/master/docs/iris/src/conf.py