diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
index 2176b8260c5c..03b879699957 100644
--- a/compiler/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
@@ -945,7 +945,7 @@ object desugar {
       report.error(IllegalRedefinitionOfStandardKind(kind, name), errPos)
       name = name.errorName
     }
-    if name.isExtensionName && !mdef.mods.is(ExtensionMethod) then
+    if name.isExtensionName && (!mdef.mods.is(ExtensionMethod) || name.dropExtension.isExtensionName) then
       report.error(em"illegal method name: $name may not start with `extension_`", errPos)
     name
   }
diff --git a/tests/neg/illegal-extension.scala b/tests/neg/illegal-extension.scala
index 5adbc87b8f18..e1d58755f1ff 100644
--- a/tests/neg/illegal-extension.scala
+++ b/tests/neg/illegal-extension.scala
@@ -1,3 +1,5 @@
 trait A {
   def extension_n: String = "illegal method" // error: illegal method name: extension_n may not start with `extension_`
 }
+
+extension (x: Any) def extension_foo: String = "foo" // error: illegal method name: extension_foo may not start with `extension_`