diff --git a/docs/src/_static/theme_override.css b/docs/src/_static/theme_override.css
index 5edc286630..c56b720f69 100644
--- a/docs/src/_static/theme_override.css
+++ b/docs/src/_static/theme_override.css
@@ -26,3 +26,17 @@ table.docutils td {
     word-wrap: break-word;
 }
 
+/* Used for very strong warning */
+#slim-red-box-message {
+  background: #ff0000;
+  box-sizing: border-box;
+  color: #ffffff;
+  font-weight: normal;
+  padding: 0.5em;
+}
+
+#slim-red-box-message a {
+  color: #ffffff;
+  font-weight: normal;  
+  text-decoration:underline;
+}
diff --git a/docs/src/_templates/layout.html b/docs/src/_templates/layout.html
index 9b4983697e..96a2e0913e 100644
--- a/docs/src/_templates/layout.html
+++ b/docs/src/_templates/layout.html
@@ -1,5 +1,27 @@
 {% extends "!layout.html" %}
 
+{# This uses blocks.  See: 
+     https://www.sphinx-doc.org/en/master/templating.html
+#}
+
+/*---------------------------------------------------------------------------*/
+
+{%- block document %}
+    {% if READTHEDOCS and rtd_version == 'latest' %}
+        <div id="slim-red-box-message">
+            You are viewing the <b>latest</b> unreleased documentation
+            <b>v{{ version }}</b>.  You may prefer a 
+            <a href="https://scitools-iris.readthedocs.io/en/stable/">stable</a>
+            version.
+        </div>
+        <p></p>
+    {%- endif %}
+
+    {{ super() }}
+{%- endblock %}
+
+/*-----------------------------------------------------z----------------------*/
+
 {% block menu %}
     {{ super() }}
 
diff --git a/docs/src/common_links.inc b/docs/src/common_links.inc
index 3c465b67dc..23a933367e 100644
--- a/docs/src/common_links.inc
+++ b/docs/src/common_links.inc
@@ -24,6 +24,7 @@
 .. _New Issue: https://github.com/scitools/iris/issues/new/choose
 .. _pull request: https://github.com/SciTools/iris/pulls
 .. _pull requests: https://github.com/SciTools/iris/pulls
+.. _Read the Docs: https://scitools-iris.readthedocs.io/en/latest/
 .. _readthedocs.yml: https://github.com/SciTools/iris/blob/master/requirements/ci/readthedocs.yml
 .. _SciTools: https://github.com/SciTools
 .. _sphinx: https://www.sphinx-doc.org/en/master/
diff --git a/docs/src/conf.py b/docs/src/conf.py
index 9bab5850b8..ab05312fca 100644
--- a/docs/src/conf.py
+++ b/docs/src/conf.py
@@ -43,6 +43,9 @@ def autolog(message):
     for item, value in os.environ.items():
         autolog("[READTHEDOCS] {} = {}".format(item, value))
 
+# This is the rtd reference to the version, such as: latest, stable, v3.0.1 etc
+# For local testing purposes this could be explicitly set latest or stable.
+rtd_version = os.environ.get("READTHEDOCS_VERSION")
 
 # -- Path setup --------------------------------------------------------------
 
@@ -131,7 +134,6 @@ def autolog(message):
     "custom_data_autodoc",
     "generate_package_rst",
 ]
-
 # -- panels extension ---------------------------------------------------------
 # See https://sphinx-panels.readthedocs.io/en/latest/
 
@@ -165,7 +167,7 @@ def autolog(message):
 # See https://sphinx-copybutton.readthedocs.io/en/latest/
 copybutton_prompt_text = ">>> "
 
-# sphinx.ext.todo configuration
+# sphinx.ext.todo configuration -----------------------------------------------
 # See https://www.sphinx-doc.org/en/master/usage/extensions/todo.html
 todo_include_todos = True
 
@@ -228,6 +230,8 @@ def autolog(message):
 }
 
 html_context = {
+    "rtd_version": rtd_version,
+    "version": version,
     "copyright_years": copyright_years,
     "python_version": build_python_version,
     # menu_links and menu_links_name are used in _templates/layout.html
@@ -296,7 +300,6 @@ def autolog(message):
     "ignore_pattern": r"__init__\.py",
 }
 
-
 # -----------------------------------------------------------------------------
 # Remove matplotlib agg warnings from generated doc when using plt.show
 warnings.filterwarnings(
@@ -306,7 +309,6 @@ def autolog(message):
     " non-GUI backend, so cannot show the figure.",
 )
 
-
 # -- numfig options (built-in) ------------------------------------------------
 # Enable numfig.
 numfig = True
diff --git a/docs/src/whatsnew/latest.rst b/docs/src/whatsnew/latest.rst
index a70385a7ed..538a38da9b 100644
--- a/docs/src/whatsnew/latest.rst
+++ b/docs/src/whatsnew/latest.rst
@@ -91,6 +91,10 @@ This document explains the changes made to Iris for this release
 
 #. `@bjlittle`_ added the |PyPI|_ badge to the `README.md`_. (:pull:`4004`)
 
+#. `@tkknight`_ added a banner at the top of every page of the unreleased
+   development documentation if being viewed on `Read the Docs`_.
+   (:pull:`3999`)
+
 
 💼 Internal
 ===========