The British government will invoke Article 50 of the EU's Lisbon Treaty, which says a member state may "notify the European Council of its intention" to leave the bloc.

The Department for Exiting the European Union says notification will come in a letter from Prime Minister Theresa May to European Council President Donald Tusk. May will also announce the news in Parliament.

That sets a clock ticking: Article 50 says that two years from the moment of notification, "the Treaties shall cease to apply" and Britain will no longer be an EU member.