Index: cp/decl.c
===================================================================
--- cp/decl.c	(revision 167817)
+++ cp/decl.c	(working copy)
@@ -9210,6 +9210,12 @@ grokdeclarator (const cp_declarator *dec
 	  error ("const %qs cannot be declared %<mutable%>", name);
 	  storage_class = sc_none;
 	}
+      else if (TREE_CODE (type) == REFERENCE_TYPE)
+	{
+	  permerror (input_location, "reference %qs cannot be declared "
+	             "%<mutable%>", name);
+	  storage_class = sc_none;
+	}
     }
 
   /* If this is declaring a typedef name, return a TYPE_DECL.  */
Index: testsuite/g++.dg/other/pr33558.C
===================================================================
--- testsuite/g++.dg/other/pr33558.C	(revision 0)
+++ testsuite/g++.dg/other/pr33558.C	(revision 0)
@@ -0,0 +1,5 @@
+/* { dg-do compile } */
+
+class X {
+  mutable int &q; /* { dg-error "cannot be declared 'mutable'" } */
+};
Index: testsuite/g++.dg/other/pr33558-2.C
===================================================================
--- testsuite/g++.dg/other/pr33558-2.C	(revision 0)
+++ testsuite/g++.dg/other/pr33558-2.C	(revision 0)
@@ -0,0 +1,6 @@
+/* { dg-do compile } */
+/* { dg-options "-fpermissive" } */
+
+class X {
+  mutable int &q; /* { dg-warning "cannot be declared 'mutable'" } */
+};