diff --git a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
index 1058b723552..cafaf68df2d 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
@@ -162,6 +162,7 @@
 * Direct delegate construction ([PR ##19993](https://github.com/dotnet/fsharp/pull/19993))
 
 ### Changed
+* The `--warnaserror` option now ignores unrecognized diagnostic identifiers in warning lists while still applying recognized F# warning codes. ([PR #20246](https://github.com/dotnet/fsharp/pull/20246))
 
 * Improvements in error and warning messages: new error FS3885 when `let!`/`use!` is the final expression in a computation expression; new warning FS3886 when a list literal contains a single tuple element (likely missing `;` separator); improved wording for FS0003, FS0025, FS0039, FS0072, FS0247, FS0597, FS0670, FS3082, and SRTP operator-not-in-scope hints. ([PR #19398](https://github.com/dotnet/fsharp/pull/19398))
 * Exception field serialization (`GetObjectData` and field-restoring constructor) is now gated behind `langversion:11` (`LanguageFeature.ExceptionFieldSerializationSupport`). With langversion ≤10, exception codegen is unchanged from pre-#19342 behavior. ([PR #19746](https://github.com/dotnet/fsharp/pull/19746))
diff --git a/src/Compiler/Driver/CompilerOptions.fs b/src/Compiler/Driver/CompilerOptions.fs
index 48574325813..bf6e72f0d95 100644
--- a/src/Compiler/Driver/CompilerOptions.fs
+++ b/src/Compiler/Driver/CompilerOptions.fs
@@ -791,14 +791,6 @@ let inputFileFlagsFsi (tcConfigB: TcConfigBuilder) =
 //---------------------------------
 
 let errorsAndWarningsFlags (tcConfigB: TcConfigBuilder) =
-    let trimFS (s: string) =
-        if s.StartsWithOrdinal "FS" then s.Substring 2 else s
-
-    let trimFStoInt (s: string) =
-        match Int32.TryParse(trimFS s) with
-        | true, n -> Some n
-        | false, _ -> None
-
     [
         CompilerOption(
             "warnaserror",
@@ -816,7 +808,14 @@ let errorsAndWarningsFlags (tcConfigB: TcConfigBuilder) =
             "warnaserror",
             tagWarnList,
             OptionStringListSwitch(fun n switch ->
-                match trimFStoInt n with
+                match
+                    GetWarningNumber(
+                        rangeCmdArgs,
+                        WarningDescription.String n,
+                        tcConfigB.langVersion,
+                        WarningNumberSource.CommandLineOption
+                    )
+                with
                 | Some n ->
                     let options = tcConfigB.diagnosticsOptions
 
diff --git a/tests/FSharp.Compiler.ComponentTests/CompilerDirectives/NonStringArgs.fs b/tests/FSharp.Compiler.ComponentTests/CompilerDirectives/NonStringArgs.fs
index a8b275ab3d5..47e5e5109b9 100644
--- a/tests/FSharp.Compiler.ComponentTests/CompilerDirectives/NonStringArgs.fs
+++ b/tests/FSharp.Compiler.ComponentTests/CompilerDirectives/NonStringArgs.fs
@@ -143,6 +143,14 @@ match None with None -> ()      // creates FS0025 - ignored due to flag
                 (Warning 988, Line 3, Col 3, Line 3, Col 3, "Main module of program is empty: nothing will happen when it is run")
             ]
 
+    [<Fact>]
+    let ``--warnaserror ignores unknown diagnostic identifiers`` () =
+        FSharp """ "" """
+        |> withOptions ["--warnaserror:NU1605;FS20"]
+        |> typecheck
+        |> shouldFail
+        |> withErrorCode 20
+
 
     [<InlineData("8.0")>]
     [<InlineData("9.0")>]