In brief: leaving the European Union
The Lisbon Treaty allows EU Member States to leave the European Union. However, the process, as yet untested, would in all likelihood not be straightforward and would involve negotiating a new relationship for the UK with the EU.
The Treaty of Lisbon provided for Member States to leave the EU if they wanted to. Article 50 of the Treaty on European Union (TEU), as amended by the Lisbon Treaty, sets out a procedure for a voluntary withdrawal from the Union according to a State’s “own constitutional requirements”.
Withdrawing from the EU would not be easy, however, and would not mean a simple return to the status quo before the UK joined the then European Economic Community (EEC) in 1973. EU law is part of UK law and its enactment has given UK citizens, companies and state authorities certain rights and obligations; changing or removing them would not be straightforward.
A number of complex issues would need to be resolved through negotiations with the other EU Member States. These would include a new relationship with the Common Agricultural and Common Fisheries Policies, revised trade rules with EU Member States and with third parties, changes to the arrangements for the free movement of workers throughout the EU and EEA areas, to name but a few. The UK would probably negotiate transitional arrangements to take account of these and other matters, and then establish a new relationship with the EU.