diff --git a/developer/src/common/include/kmn_compiler_errors.h b/developer/src/common/include/kmn_compiler_errors.h
index fe7ba11a150..e0bea39549c 100644
--- a/developer/src/common/include/kmn_compiler_errors.h
+++ b/developer/src/common/include/kmn_compiler_errors.h
@@ -240,6 +240,8 @@
 
 #define CHINT_UnreachableRule                              0x000010AE
 
+#define CWARN_VirtualKeyInOutput                           0x000020AF
+
 #define CERR_BufferOverflow                                0x000080C0
 #define CERR_Break                                         0x000080C1
 
diff --git a/developer/src/kmc-kmn/src/compiler/kmn-compiler-messages.ts b/developer/src/kmc-kmn/src/compiler/kmn-compiler-messages.ts
index 2995c8f0165..4ad9d025982 100644
--- a/developer/src/kmc-kmn/src/compiler/kmn-compiler-messages.ts
+++ b/developer/src/kmc-kmn/src/compiler/kmn-compiler-messages.ts
@@ -302,6 +302,8 @@ export class KmnCompilerMessages {
 
   static HINT_UnreachableRule                                 = SevHint | 0x0AE;
 
+  static WARN_VirtualKeyInOutput                              = SevWarn | 0x0AF;
+
   static FATAL_BufferOverflow                                 = SevFatal | 0x0C0;
   static FATAL_Break                                          = SevFatal | 0x0C1;
 };
diff --git a/developer/src/kmc-kmn/test/fixtures/invalid-keyboards/warn_virtual_key_in_output.kmn b/developer/src/kmc-kmn/test/fixtures/invalid-keyboards/warn_virtual_key_in_output.kmn
new file mode 100644
index 00000000000..e4919689b10
--- /dev/null
+++ b/developer/src/kmc-kmn/test/fixtures/invalid-keyboards/warn_virtual_key_in_output.kmn
@@ -0,0 +1,9 @@
+store(&NAME) 'WARN_VirtualKeyInOutput'
+store(&VERSION) '9.0'
+
+begin Unicode > use(main)
+
+group(main) using keys
+
+c WARN_VirtualKeyInOutput
++ 'a' > [K_BKQUOTE]
diff --git a/developer/src/kmc-kmn/test/test-messages.ts b/developer/src/kmc-kmn/test/test-messages.ts
index 6970b997417..51e6894daa7 100644
--- a/developer/src/kmc-kmn/test/test-messages.ts
+++ b/developer/src/kmc-kmn/test/test-messages.ts
@@ -87,4 +87,11 @@ describe('CompilerMessages', function () {
     assert.equal(callbacks.messages[0].message, "Statement 'return' is not currently supported in output for web and touch targets");
   });
 
+  // WARN_VirtualKeyInOutput
+
+  it('should generate WARN_VirtualKeyInOutput if a virtual key is found in the output part of a rule', async function() {
+    await testForMessage(this, ['invalid-keyboards', 'warn_virtual_key_in_output.kmn'], KmnCompilerMessages.WARN_VirtualKeyInOutput);
+    assert.equal(callbacks.messages[0].message, "Virtual keys are not supported in output");
+  });
+
 });
diff --git a/developer/src/kmcmplib/src/CompMsg.cpp b/developer/src/kmcmplib/src/CompMsg.cpp
index 8ab5db81f6b..715750d4f2f 100644
--- a/developer/src/kmcmplib/src/CompMsg.cpp
+++ b/developer/src/kmcmplib/src/CompMsg.cpp
@@ -143,6 +143,7 @@ const struct CompilerError CompilerErrors[] = {
     { CWARN_NulNotFirstStatementInContext                , "nul must be the first statement in the context"},
     { CWARN_IfShouldBeAtStartOfContext                   , "if, platform and baselayout should be at start of context (after nul, if present)"},
     { CWARN_KeyShouldIncludeNCaps                        , "Other rules which reference this key include CAPS or NCAPS modifiers, so this rule must include NCAPS modifier to avoid inconsistent matches"},
+    { CWARN_VirtualKeyInOutput                           , "Virtual keys are not supported in output"},
 
     { 0, nullptr }
   };
diff --git a/developer/src/kmcmplib/src/Compiler.cpp b/developer/src/kmcmplib/src/Compiler.cpp
index e2720f8608e..045b1c39ad1 100644
--- a/developer/src/kmcmplib/src/Compiler.cpp
+++ b/developer/src/kmcmplib/src/Compiler.cpp
@@ -1331,6 +1331,7 @@ KMX_BOOL CheckContextStatementPositions(PKMX_WCHAR context) {
 
   return TRUE;
 }
+
 /**
  *  Checks if a use() statement is followed by other content in the output of a rule
  */
@@ -1348,6 +1349,22 @@ KMX_DWORD CheckUseStatementsInOutput(PKMX_WCHAR output) {
   return CERR_None;
 }
 
+/**
+ * Warn if output has virtual keys in it, which is not supported by Core at all,
+ * but was unofficially supported, but never worked properly, in Keyman for
+ * Windows for many years
+ */
+KMX_DWORD CheckVirtualKeysInOutput(PKMX_WCHAR output) {
+  PKMX_WCHAR p;
+  for (p = output; *p; p = incxstr(p)) {
+    if (*p == UC_SENTINEL && *(p + 1) == CODE_EXTENDED) {
+      AddWarning(CWARN_VirtualKeyInOutput);
+      break;
+    }
+  }
+  return CERR_None;
+}
+
 /**
  * Adds implicit `context` to start of output of rules for readonly groups
  */
@@ -1472,6 +1489,11 @@ KMX_DWORD ProcessKeyLineImpl(PFILE_KEYBOARD fk, PKMX_WCHAR str, KMX_BOOL IsUnico
     return msg;   // I4867
   }
 
+  // Warn if virtual keys are used in the output, as they are unsupported by Core
+  if ((msg = CheckVirtualKeysInOutput(pklOut)) != CERR_None) {
+    return msg;
+  }
+
   if (gp->fReadOnly) {
     // Ensure no output is made from the rule, and that
     // use() statements meet required readonly semantics