diff --git a/release_docs b/release_docs
index 95ebf52ad..8367d81f1 100755
--- a/release_docs
+++ b/release_docs
@@ -9,6 +9,12 @@ fi
 
 version=`python -c "import cfdm; print(cfdm.__version__)"`
 
+sphinx_version=`python -c "import sphinx; print(sphinx.__version__)"`
+if [[ $sphinx_version != "2.4.5"  ]] ; then
+  echo "ERROR: Must use sphinx version 2.4.5. Got $sphinx_version"
+  exit 3
+fi
+
 if [[ $1 = "latest" ]] ; then 
   dir=$PWD/docs
 elif [[ $1 = "archive" ]] ; then