diff --git a/java/core/src/test/proto/com/google/protobuf/test_bad_identifiers.proto b/java/core/src/test/proto/com/google/protobuf/test_bad_identifiers.proto
index 58665e0486e51..bf8e71ad3aacc 100644
--- a/java/core/src/test/proto/com/google/protobuf/test_bad_identifiers.proto
+++ b/java/core/src/test/proto/com/google/protobuf/test_bad_identifiers.proto
@@ -177,6 +177,9 @@ service TestConflictingMethodNames {
 }
 
 message TestConflictingFieldNames {
+  // TODO(b/261750190) Remove these tests once this behavior is removed.
+  option deprecated_legacy_json_field_conflicts = true;
+
   enum TestEnum {
     UNKNOWN = 0;
     FOO = 1;
diff --git a/python/google/protobuf/descriptor_pool.py b/python/google/protobuf/descriptor_pool.py
index dccac5e714418..2523bbb6c1db0 100644
--- a/python/google/protobuf/descriptor_pool.py
+++ b/python/google/protobuf/descriptor_pool.py
@@ -120,11 +120,25 @@ class DescriptorPool(object):
 
   if _USE_C_DESCRIPTORS:
 
-    def __new__(cls, descriptor_db=None):
-      # pylint: disable=protected-access
-      return descriptor._message.DescriptorPool(descriptor_db)
+    def __new__(
+        cls,
+        descriptor_db=None,
+        use_deprecated_legacy_json_field_conflicts=False,
+    ):
+      if descriptor.api_implementation.Type() == 'cpp':
+        # TODO(b/261750190) Remove this once ranklab no longer needs it.
+        # pylint: disable=protected-access
+        return descriptor._message.DescriptorPool(
+            descriptor_db,
+            use_deprecated_legacy_json_field_conflicts=use_deprecated_legacy_json_field_conflicts,
+        )
+      else:
+        # pylint: disable=protected-access
+        return descriptor._message.DescriptorPool(descriptor_db)
 
-  def __init__(self, descriptor_db=None):
+  def __init__(
+      self, descriptor_db=None, use_deprecated_legacy_json_field_conflicts=False
+  ):
     """Initializes a Pool of proto buffs.
 
     The descriptor_db argument to the constructor is provided to allow
@@ -135,6 +149,8 @@ def __init__(self, descriptor_db=None):
 
     Args:
       descriptor_db: A secondary source of file descriptors.
+      use_deprecated_legacy_json_field_conflicts: Unused, for compatibility with
+        C++.
     """
 
     self._internal_db = descriptor_database.DescriptorDatabase()
diff --git a/python/google/protobuf/internal/descriptor_test.py b/python/google/protobuf/internal/descriptor_test.py
index 614a84580d701..b070bca87dc20 100755
--- a/python/google/protobuf/internal/descriptor_test.py
+++ b/python/google/protobuf/internal/descriptor_test.py
@@ -1186,6 +1186,7 @@ def testMakeDescriptorWithOptions(self):
 
   def testCamelcaseName(self):
     descriptor_proto = descriptor_pb2.DescriptorProto()
+    descriptor_proto.options.deprecated_legacy_json_field_conflicts = True
     descriptor_proto.name = 'Bar'
     names = ['foo_foo', 'FooBar', 'fooBaz', 'fooFoo', 'foobar']
     camelcase_names = ['fooFoo', 'fooBar', 'fooBaz', 'fooFoo', 'foobar']
@@ -1200,6 +1201,7 @@ def testCamelcaseName(self):
 
   def testJsonName(self):
     descriptor_proto = descriptor_pb2.DescriptorProto()
+    descriptor_proto.options.deprecated_legacy_json_field_conflicts = True
     descriptor_proto.name = 'TestJsonName'
     names = ['field_name', 'fieldName', 'FieldName',
              '_field_name', 'FIELD_NAME', 'json_name']
diff --git a/python/google/protobuf/pyext/descriptor_pool.cc b/python/google/protobuf/pyext/descriptor_pool.cc
index 9388f16c960a5..5f80c9eb79e7a 100644
--- a/python/google/protobuf/pyext/descriptor_pool.cc
+++ b/python/google/protobuf/pyext/descriptor_pool.cc
@@ -155,20 +155,26 @@ static PyDescriptorPool* PyDescriptorPool_NewWithUnderlay(
 }
 
 static PyDescriptorPool* PyDescriptorPool_NewWithDatabase(
-    DescriptorDatabase* database) {
+    DescriptorDatabase* database,
+    bool use_deprecated_legacy_json_field_conflicts) {
   PyDescriptorPool* cpool = _CreateDescriptorPool();
   if (cpool == nullptr) {
     return nullptr;
   }
+  DescriptorPool* pool;
   if (database != nullptr) {
     cpool->error_collector = new BuildFileErrorCollector();
-    cpool->pool = new DescriptorPool(database, cpool->error_collector);
+    pool = new DescriptorPool(database, cpool->error_collector);
     cpool->is_mutable = false;
     cpool->database = database;
   } else {
-    cpool->pool = new DescriptorPool();
+    pool = new DescriptorPool();
     cpool->is_mutable = true;
   }
+  if (use_deprecated_legacy_json_field_conflicts) {
+    pool->UseDeprecatedLegacyJsonFieldConflicts();
+  }
+  cpool->pool = pool;
   cpool->is_owned = true;
 
   if (!descriptor_pool_map->insert(std::make_pair(cpool->pool, cpool)).second) {
@@ -183,18 +189,21 @@ static PyDescriptorPool* PyDescriptorPool_NewWithDatabase(
 // The public DescriptorPool constructor.
 static PyObject* New(PyTypeObject* type,
                      PyObject* args, PyObject* kwargs) {
-  static const char* kwlist[] = {"descriptor_db", nullptr};
+  static const char* kwlist[] = {
+      "descriptor_db", "use_deprecated_legacy_json_field_conflicts", nullptr};
   PyObject* py_database = nullptr;
-  if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|O",
-                                   const_cast<char**>(kwlist), &py_database)) {
+  int use_deprecated_legacy_json_field_conflicts = 0;
+  if (!PyArg_ParseTupleAndKeywords(
+          args, kwargs, "|O$i", const_cast<char**>(kwlist), &py_database,
+          &use_deprecated_legacy_json_field_conflicts)) {
     return nullptr;
   }
   DescriptorDatabase* database = nullptr;
   if (py_database && py_database != Py_None) {
     database = new PyDescriptorDatabase(py_database);
   }
-  return reinterpret_cast<PyObject*>(
-      PyDescriptorPool_NewWithDatabase(database));
+  return reinterpret_cast<PyObject*>(PyDescriptorPool_NewWithDatabase(
+      database, use_deprecated_legacy_json_field_conflicts));
 }
 
 static void Dealloc(PyObject* pself) {
diff --git a/src/google/protobuf/descriptor.cc b/src/google/protobuf/descriptor.cc
index f914711b42af1..9c3c9b9e1ad3c 100644
--- a/src/google/protobuf/descriptor.cc
+++ b/src/google/protobuf/descriptor.cc
@@ -5617,16 +5617,8 @@ void DescriptorBuilder::CheckFieldJsonNameUniqueness(
         this_type, field.name(), details.orig_name, existing_type,
         match.field->name(), name_suffix);
 
-    bool involves_default = !details.is_custom || !match.is_custom;
-    if (syntax == FileDescriptor::SYNTAX_PROTO2 && involves_default) {
-      // TODO(b/261750676) Upgrade this to an error once downstream proto2 files
-      // have been fixed.
-      AddWarning(message_name, field, DescriptorPool::ErrorCollector::NAME,
-                 error_message);
-    } else {
-      AddError(message_name, field, DescriptorPool::ErrorCollector::NAME,
-               error_message);
-    }
+    AddError(message_name, field, DescriptorPool::ErrorCollector::NAME,
+             error_message);
   }
 }
 
diff --git a/src/google/protobuf/descriptor_unittest.cc b/src/google/protobuf/descriptor_unittest.cc
index c1fa463932195..a82d8f812e2dd 100644
--- a/src/google/protobuf/descriptor_unittest.cc
+++ b/src/google/protobuf/descriptor_unittest.cc
@@ -1277,6 +1277,8 @@ class StylizedFieldNamesTest : public testing::Test {
     AddExtensionRange(AddMessage(&file, "ExtendableMessage"), 1, 1000);
 
     DescriptorProto* message = AddMessage(&file, "TestMessage");
+    message->mutable_options()->set_deprecated_legacy_json_field_conflicts(
+        true);
     AddField(message, "foo_foo", 1, FieldDescriptorProto::LABEL_OPTIONAL,
              FieldDescriptorProto::TYPE_INT32);
     AddField(message, "FooBar", 2, FieldDescriptorProto::LABEL_OPTIONAL,
@@ -7012,7 +7014,7 @@ TEST_F(ValidationErrorTest, ValidateJsonNameConflictProto3) {
 }
 
 TEST_F(ValidationErrorTest, ValidateJsonNameConflictProto2) {
-  BuildFileWithWarnings(
+  BuildFileWithErrors(
       "name: 'foo.proto' "
       "syntax: 'proto2' "
       "message_type {"