diff --git a/flang/docs/Extensions.md b/flang/docs/Extensions.md
--- a/flang/docs/Extensions.md
+++ b/flang/docs/Extensions.md
@@ -346,6 +346,9 @@
 * User (non-intrinsic) `ELEMENTAL` procedures may not be passed as actual
   arguments, in accordance with the standard; some Fortran compilers
   permit such usage.
+* Constraint C1406, which prohibits the same module name from being used
+  in a scope for both an intrinsic and a non-intrinsic module, is implemented
+  as a portability warning only, not a hard error.
 
 ## Preprocessing behavior
 
diff --git a/flang/lib/Semantics/resolve-names.cpp b/flang/lib/Semantics/resolve-names.cpp
--- a/flang/lib/Semantics/resolve-names.cpp
+++ b/flang/lib/Semantics/resolve-names.cpp
@@ -740,12 +740,12 @@
   bool BeginSubmodule(const parser::Name &, const parser::ParentIdentifier &);
   void ApplyDefaultAccess();
   Symbol &AddGenericUse(GenericDetails &, const SourceName &, const Symbol &);
-  void AddAndCheckExplicitIntrinsicUse(SourceName, bool isIntrinsic);
+  void AddAndCheckModuleUse(SourceName, bool isIntrinsic);
   void ClearUseRenames() { useRenames_.clear(); }
   void ClearUseOnly() { useOnly_.clear(); }
-  void ClearExplicitIntrinsicUses() {
-    explicitIntrinsicUses_.clear();
-    explicitNonIntrinsicUses_.clear();
+  void ClearModuleUses() {
+    intrinsicUses_.clear();
+    nonIntrinsicUses_.clear();
   }
 
 private:
@@ -759,10 +759,10 @@
   std::set<std::pair<SourceName, Scope *>> useRenames_;
   // Names that have appeared in an ONLY clause of a USE statement
   std::set<std::pair<SourceName, Scope *>> useOnly_;
-  // Module names that have appeared in USE statements with explicit
-  // INTRINSIC or NON_INTRINSIC keywords
-  std::set<SourceName> explicitIntrinsicUses_;
-  std::set<SourceName> explicitNonIntrinsicUses_;
+  // Intrinsic and non-intrinsic (explicit or not) module names that
+  // have appeared in USE statements; used for C1406 warnings.
+  std::set<SourceName> intrinsicUses_;
+  std::set<SourceName> nonIntrinsicUses_;
 
   Symbol &SetAccess(const SourceName &, Attr attr, Symbol * = nullptr);
   // A rename in a USE statement: local => use
@@ -2748,7 +2748,6 @@
   std::optional<bool> isIntrinsic;
   if (x.nature) {
     isIntrinsic = *x.nature == parser::UseStmt::ModuleNature::Intrinsic;
-    AddAndCheckExplicitIntrinsicUse(x.moduleName.source, *isIntrinsic);
   } else if (currScope().IsModule() && currScope().symbol() &&
       currScope().symbol()->attrs().test(Attr::INTRINSIC)) {
     // Intrinsic modules USE only other intrinsic modules
@@ -2758,6 +2757,8 @@
   if (!useModuleScope_) {
     return false;
   }
+  AddAndCheckModuleUse(x.moduleName.source,
+      useModuleScope_->parent().kind() == Scope::Kind::IntrinsicModules);
   // use the name from this source file
   useModuleScope_->symbol()->ReplaceName(x.moduleName.source);
   return true;
@@ -3062,27 +3063,25 @@
   return newSymbol;
 }
 
-// Enforce C1406
-void ModuleVisitor::AddAndCheckExplicitIntrinsicUse(
-    SourceName name, bool isIntrinsic) {
+// Enforce C1406 as a warning
+void ModuleVisitor::AddAndCheckModuleUse(SourceName name, bool isIntrinsic) {
   if (isIntrinsic) {
-    if (auto iter{explicitNonIntrinsicUses_.find(name)};
-        iter != explicitNonIntrinsicUses_.end()) {
+    if (auto iter{nonIntrinsicUses_.find(name)};
+        iter != nonIntrinsicUses_.end()) {
       Say(name,
-          "Cannot USE,INTRINSIC module '%s' in the same scope as USE,NON_INTRINSIC"_err_en_US,
+          "Should not USE the intrinsic module '%s' in the same scope as a USE of the non-intrinsic module"_port_en_US,
           name)
           .Attach(*iter, "Previous USE of '%s'"_en_US, *iter);
     }
-    explicitIntrinsicUses_.insert(name);
+    intrinsicUses_.insert(name);
   } else {
-    if (auto iter{explicitIntrinsicUses_.find(name)};
-        iter != explicitIntrinsicUses_.end()) {
+    if (auto iter{intrinsicUses_.find(name)}; iter != intrinsicUses_.end()) {
       Say(name,
-          "Cannot USE,NON_INTRINSIC module '%s' in the same scope as USE,INTRINSIC"_err_en_US,
+          "Should not USE the non-intrinsic module '%s' in the same scope as a USE of the intrinsic module"_port_en_US,
           name)
           .Attach(*iter, "Previous USE of '%s'"_en_US, *iter);
     }
-    explicitNonIntrinsicUses_.insert(name);
+    nonIntrinsicUses_.insert(name);
   }
 }
 
@@ -7406,7 +7405,7 @@
   Walk(useStmts);
   ClearUseRenames();
   ClearUseOnly();
-  ClearExplicitIntrinsicUses();
+  ClearModuleUses();
   Walk(importStmts);
   Walk(implicitPart);
   for (const auto &decl : decls) {
diff --git a/flang/test/Semantics/modfile43.f90 b/flang/test/Semantics/modfile43.f90
--- a/flang/test/Semantics/modfile43.f90
+++ b/flang/test/Semantics/modfile43.f90
@@ -5,7 +5,7 @@
 end module
 module m1
   use, intrinsic :: iso_fortran_env, only: int32
-  !ERROR: Cannot USE,NON_INTRINSIC module 'iso_fortran_env' in the same scope as USE,INTRINSIC
+  !PORTABILITY: Should not USE the non-intrinsic module 'iso_fortran_env' in the same scope as a USE of the intrinsic module
   use, non_intrinsic :: iso_fortran_env, only: user_defined_123
 end module
 module m2