Leaving the European Union by the United Kingdom will cause changes in the Treaty on the European Union as well as in the Treaty on the Functioning of the European Union. These changes are covered by the ordinary legislative procedure which includes the necessity of their ratification by Poland. The Sejm may take a position on Brexit, however it should take into account the role of the Council of Ministers in foreign policy. The result of UK’s withdrawal from the EU will be a considerable number of changes in the Polish legislation. Due to their scope as well as short time constraints, work on these issues should be held parallelly with negotiations on Brexit.