diff --git a/docs/src/whatsnew/latest.rst b/docs/src/whatsnew/latest.rst
index c2bec897e1..2976ae56ca 100644
--- a/docs/src/whatsnew/latest.rst
+++ b/docs/src/whatsnew/latest.rst
@@ -125,6 +125,12 @@ This document explains the changes made to Iris for this release
    to be compatible with GeoVista v0.5, as well as previous versions.
    (:pull:`6064`)
 
+#. `@pp-mo`_ temporarily pinned matplotlib to ">=3.5, !=3.9.1", to avoid current CI
+   test failures on plot results, apparently due to a matplotlib bug.
+   See : https://github.com/matplotlib/matplotlib/issues/28567
+   (:pull:`6065`)
+
+
 
 📚 Documentation
 ================
diff --git a/requirements/py310.yml b/requirements/py310.yml
index f7184cc769..309f8aa9a2 100644
--- a/requirements/py310.yml
+++ b/requirements/py310.yml
@@ -16,7 +16,7 @@ dependencies:
   - cftime >=1.5
   - dask-core >=2022.9.0
   - libnetcdf !=4.9.1
-  - matplotlib-base >=3.5
+  - matplotlib-base >=3.5, !=3.9.1
   - netcdf4
   - numpy >=1.24, !=1.24.3, <2
   - python-xxhash
diff --git a/requirements/py311.yml b/requirements/py311.yml
index 85d9d9271e..58d71ddd52 100644
--- a/requirements/py311.yml
+++ b/requirements/py311.yml
@@ -16,7 +16,7 @@ dependencies:
   - cftime >=1.5
   - dask-core >=2022.9.0
   - libnetcdf !=4.9.1
-  - matplotlib-base >=3.5
+  - matplotlib-base >=3.5, !=3.9.1
   - netcdf4
   - numpy >=1.24, !=1.24.3, <2
   - python-xxhash
diff --git a/requirements/py312.yml b/requirements/py312.yml
index 02622c3dc1..e1e62e52d9 100644
--- a/requirements/py312.yml
+++ b/requirements/py312.yml
@@ -16,7 +16,7 @@ dependencies:
   - cftime >=1.5
   - dask-core >=2022.9.0
   - libnetcdf !=4.9.1
-  - matplotlib-base >=3.5
+  - matplotlib-base >=3.5, !=3.9.1
   - netcdf4
   - numpy >=1.24, !=1.24.3, <2
   - python-xxhash