diff --git a/src/sed/compiler.rs b/src/sed/compiler.rs
index bcfff31..20d416d 100644
--- a/src/sed/compiler.rs
+++ b/src/sed/compiler.rs
@@ -791,7 +791,7 @@ fn compile_subst_command(
     let mut subst = Box::new(Substitution::default());
 
     subst.replacement = compile_replacement(lines, line)?;
-    compile_subst_flags(lines, line, &mut subst)?;
+    compile_subst_flags(lines, line, &mut subst, context)?;
 
     if pattern.is_empty() && subst.ignore_case {
         return compilation_error(
@@ -865,6 +865,7 @@ pub fn compile_subst_flags(
     lines: &ScriptLineProvider,
     line: &mut ScriptCharProvider,
     subst: &mut Substitution,
+    context: &mut ProcessingContext,
 ) -> UResult<()> {
     let mut seen_g_or_n = false;
 
@@ -899,10 +900,19 @@ pub fn compile_subst_flags(
             }
 
             'i' | 'I' => {
+                if context.posix {
+                    return compilation_error(lines, line, "unknown option to 's'");
+                }
                 subst.ignore_case = true;
                 line.advance();
             }
 
+            'm' | 'M' | 'e' => {
+                if context.posix {
+                    return compilation_error(lines, line, "unknown option to 's'");
+                }
+            }
+
             _c @ '1'..='9' => {
                 if seen_g_or_n {
                     return compilation_error(
@@ -2033,7 +2043,7 @@ mod tests {
         let (lines, mut chars) = make_providers("g");
         let mut subst = Substitution::default();
 
-        compile_subst_flags(&lines, &mut chars, &mut subst).unwrap();
+        compile_subst_flags(&lines, &mut chars, &mut subst, &mut ctx()).unwrap();
         assert_eq!(subst.occurrence, 0); // 'g' means all occurrences
     }
 
@@ -2042,7 +2052,7 @@ mod tests {
         let (lines, mut chars) = make_providers("p");
         let mut subst = Substitution::default();
 
-        compile_subst_flags(&lines, &mut chars, &mut subst).unwrap();
+        compile_subst_flags(&lines, &mut chars, &mut subst, &mut ctx()).unwrap();
         assert!(subst.print_flag);
     }
 
@@ -2051,7 +2061,7 @@ mod tests {
         let (lines, mut chars) = make_providers("I");
         let mut subst = Substitution::default();
 
-        compile_subst_flags(&lines, &mut chars, &mut subst).unwrap();
+        compile_subst_flags(&lines, &mut chars, &mut subst, &mut ctx()).unwrap();
         assert!(subst.ignore_case);
     }
 
@@ -2060,7 +2070,7 @@ mod tests {
         let (lines, mut chars) = make_providers("i");
         let mut subst = Substitution::default();
 
-        compile_subst_flags(&lines, &mut chars, &mut subst).unwrap();
+        compile_subst_flags(&lines, &mut chars, &mut subst, &mut ctx()).unwrap();
         assert!(subst.ignore_case);
     }
 
@@ -2069,7 +2079,7 @@ mod tests {
         let (lines, mut chars) = make_providers("3");
         let mut subst = Substitution::default();
 
-        compile_subst_flags(&lines, &mut chars, &mut subst).unwrap();
+        compile_subst_flags(&lines, &mut chars, &mut subst, &mut ctx()).unwrap();
         assert_eq!(subst.occurrence, 3);
     }
 
@@ -2078,7 +2088,7 @@ mod tests {
         let (lines, mut chars) = make_providers("g3");
         let mut subst = Substitution::default();
 
-        let err = compile_subst_flags(&lines, &mut chars, &mut subst).unwrap_err();
+        let err = compile_subst_flags(&lines, &mut chars, &mut subst, &mut ctx()).unwrap_err();
         assert!(
             err.to_string()
                 .contains("multiple 'g' or numeric flags in substitute command")
@@ -2090,7 +2100,7 @@ mod tests {
         let (lines, mut chars) = make_providers("2g");
         let mut subst = Substitution::default();
 
-        let err = compile_subst_flags(&lines, &mut chars, &mut subst).unwrap_err();
+        let err = compile_subst_flags(&lines, &mut chars, &mut subst, &mut ctx()).unwrap_err();
         assert!(
             err.to_string()
                 .contains("multiple 'g' or numeric flags in substitute command")
@@ -2102,7 +2112,7 @@ mod tests {
         let (lines, mut chars) = make_providers("w ");
         let mut subst = Substitution::default();
 
-        let err = compile_subst_flags(&lines, &mut chars, &mut subst).unwrap_err();
+        let err = compile_subst_flags(&lines, &mut chars, &mut subst, &mut ctx()).unwrap_err();
         assert!(err.to_string().contains("missing file path"));
     }
 
@@ -2113,7 +2123,7 @@ mod tests {
         let (lines, mut chars) = make_providers(&format!("w {}", out.display()));
         let mut subst = Substitution::default();
 
-        compile_subst_flags(&lines, &mut chars, &mut subst).unwrap();
+        compile_subst_flags(&lines, &mut chars, &mut subst, &mut ctx()).unwrap();
         assert_eq!(
             subst.write_file.as_ref().map(|w| w.borrow().path.clone()),
             Some(out)
@@ -2125,7 +2135,7 @@ mod tests {
         let (lines, mut chars) = make_providers("z");
         let mut subst = Substitution::default();
 
-        let err = compile_subst_flags(&lines, &mut chars, &mut subst).unwrap_err();
+        let err = compile_subst_flags(&lines, &mut chars, &mut subst, &mut ctx()).unwrap_err();
         assert!(err.to_string().contains("invalid substitute flag"));
     }
 
diff --git a/tests/by-util/test_sed.rs b/tests/by-util/test_sed.rs
index b31fed7..bbfee02 100644
--- a/tests/by-util/test_sed.rs
+++ b/tests/by-util/test_sed.rs
@@ -1303,3 +1303,13 @@ fn test_print_first_line_no_newline() {
         .succeeds()
         .stdout_is("foo");
 }
+
+//--posix should reject GNU substitute flags i/I https://github.com/uutils/sed/issues/401
+#[test]
+fn test_posix_reject_flags() {
+    new_ucmd!()
+        .args(&["--posix", "s/a/b/i"])
+        .fails()
+        .code_is(1)
+        .stderr_is("sed: <script argument 1>:1:7: error: unknown option to 's'\n");
+}