diff --git a/changes/1228-paul-ilyin.md b/changes/1228-paul-ilyin.md
new file mode 100644
index 0000000..76cbfc3
--- /dev/null
+++ b/changes/1228-paul-ilyin.md
@@ -0,0 +1 @@
+`update_forward_refs()` method of BaseModel now copies `__dict__` of class module instead of modyfying it
\ No newline at end of file
diff --git a/pydantic/main.py b/pydantic/main.py
index 3ace6d4..17fb874 100644
--- a/pydantic/main.py
+++ b/pydantic/main.py
@@ -646,7 +646,7 @@ class BaseModel(metaclass=ModelMetaclass):
         """
         Try to update ForwardRefs on fields based on this Model, globalns and localns.
         """
-        globalns = sys.modules[cls.__module__].__dict__
+        globalns = sys.modules[cls.__module__].__dict__.copy()
         globalns.setdefault(cls.__name__, cls)
         for f in cls.__fields__.values():
             update_field_forward_refs(f, globalns=globalns, localns=localns)
diff --git a/tests/test_aliases.py b/tests/test_aliases.py
index 751cf5e..5e0fce2 100644
--- a/tests/test_aliases.py
+++ b/tests/test_aliases.py
@@ -330,10 +330,7 @@ def test_alias_priority():
         a: str = Field(..., alias='a_field_child')
 
         class Config:
-            fields = {
-                'a': dict(alias='a_config_child'),
-                'b': dict(alias='b_config_child'),
-            }
+            fields = {'a': dict(alias='a_config_child'), 'b': dict(alias='b_config_child')}
 
             @staticmethod
             def alias_generator(x):
diff --git a/tests/test_decorator.py b/tests/test_decorator.py
index 3da5807..33acd4f 100644
--- a/tests/test_decorator.py
+++ b/tests/test_decorator.py
@@ -35,21 +35,21 @@ def test_args():
         foo(1, 'x')
 
     assert exc_info.value.errors() == [
-        {'loc': ('b',), 'msg': 'value is not a valid integer', 'type': 'type_error.integer'},
+        {'loc': ('b',), 'msg': 'value is not a valid integer', 'type': 'type_error.integer'}
     ]
 
     with pytest.raises(ValidationError) as exc_info:
         foo(1, 2, 3)
 
     assert exc_info.value.errors() == [
-        {'loc': ('args',), 'msg': '2 positional arguments expected but 3 given', 'type': 'type_error'},
+        {'loc': ('args',), 'msg': '2 positional arguments expected but 3 given', 'type': 'type_error'}
     ]
 
     with pytest.raises(ValidationError) as exc_info:
         foo(1, 2, apple=3)
 
     assert exc_info.value.errors() == [
-        {'loc': ('kwargs',), 'msg': "unexpected keyword argument: 'apple'", 'type': 'type_error'},
+        {'loc': ('kwargs',), 'msg': "unexpected keyword argument: 'apple'", 'type': 'type_error'}
     ]
 
 
@@ -87,7 +87,7 @@ def test_kwargs():
         foo(a=1, b='x')
 
     assert exc_info.value.errors() == [
-        {'loc': ('b',), 'msg': 'value is not a valid integer', 'type': 'type_error.integer'},
+        {'loc': ('b',), 'msg': 'value is not a valid integer', 'type': 'type_error.integer'}
     ]
 
     with pytest.raises(ValidationError) as exc_info:
@@ -141,7 +141,7 @@ def foo(a, b, /, c=None):
             'loc': ('v__positional_only',),
             'msg': "positional-only argument passed as keyword argument: 'b'",
             'type': 'type_error',
-        },
+        }
     ]
     with pytest.raises(ValidationError) as exc_info:
         module.foo(a=1, b=2)
@@ -150,7 +150,7 @@ def foo(a, b, /, c=None):
             'loc': ('v__positional_only',),
             'msg': "positional-only arguments passed as keyword arguments: 'a', 'b'",
             'type': 'type_error',
-        },
+        }
     ]
 
 
@@ -165,19 +165,19 @@ def test_args_name():
     with pytest.raises(ValidationError) as exc_info:
         foo(1, 2, apple=4)
     assert exc_info.value.errors() == [
-        {'loc': ('v__kwargs',), 'msg': "unexpected keyword argument: 'apple'", 'type': 'type_error'},
+        {'loc': ('v__kwargs',), 'msg': "unexpected keyword argument: 'apple'", 'type': 'type_error'}
     ]
 
     with pytest.raises(ValidationError) as exc_info:
         foo(1, 2, apple=4, banana=5)
     assert exc_info.value.errors() == [
-        {'loc': ('v__kwargs',), 'msg': "unexpected keyword arguments: 'apple', 'banana'", 'type': 'type_error'},
+        {'loc': ('v__kwargs',), 'msg': "unexpected keyword arguments: 'apple', 'banana'", 'type': 'type_error'}
     ]
 
     with pytest.raises(ValidationError) as exc_info:
         foo(1, 2, 3)
     assert exc_info.value.errors() == [
-        {'loc': ('v__args',), 'msg': '2 positional arguments expected but 3 given', 'type': 'type_error'},
+        {'loc': ('v__args',), 'msg': '2 positional arguments expected but 3 given', 'type': 'type_error'}
     ]
 
 
@@ -202,9 +202,7 @@ def test_async():
     loop.run_until_complete(run())
     with pytest.raises(ValidationError) as exc_info:
         loop.run_until_complete(foo('x'))
-    assert exc_info.value.errors() == [
-        {'loc': ('b',), 'msg': 'field required', 'type': 'value_error.missing'},
-    ]
+    assert exc_info.value.errors() == [{'loc': ('b',), 'msg': 'field required', 'type': 'value_error.missing'}]
 
 
 def test_string_annotation():
diff --git a/tests/test_main.py b/tests/test_main.py
index 1121ce6..3f69ff3 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -1,5 +1,6 @@
+import sys
 from enum import Enum
-from typing import Any, Callable, ClassVar, List, Mapping, Type
+from typing import Any, Callable, ClassVar, List, Mapping, Optional, Type
 from uuid import UUID, uuid4
 
 import pytest
@@ -995,6 +996,15 @@ def test_custom_init_subclass_params():
     assert NewModel.something == 2
 
 
+def test_update_forward_refs_does_not_modify_module_dict():
+    class MyModel(BaseModel):
+        field: Optional['MyModel']
+
+    MyModel.update_forward_refs()
+
+    assert 'MyModel' not in sys.modules[MyModel.__module__].__dict__
+
+
 def test_two_defaults():
     with pytest.raises(ValueError, match='^cannot specify both default and default_factory$'):