Britain will begin the formal process of leaving the European Union by the end of March 2017, UK Prime Minister Theresa May has announced.

The transition will likely take until 2019, May told the BBC early Sunday. The official procedure will begin when Britain invokes Article 50 of the Lisbon Treaty, and begins negotiations.

If the separation takes more than two years, Britain and the EU must unanimously agree on a lengthened split.

British citizens voted to leave the 28-country bloc that make up the EU on June 23, 2016.