On 29 March 2017, Theresa MAY, British Prime Minister, triggered Article 50 of the Lisbon Treaty.
The right of a Member State to withdraw from the European Union was introduced for the first time with the Lisbon Treaty; the possibility of withdrawal was highly controversial before that. The Article 50 provides for the negotiation of a withdrawal agreement between the EU and the withdrawing state, defining in particular the latter's future relationship with the Union. If no agreement is concluded within two years, that state's membership ends automatically, unless the European Council and the Member State concerned decide jointly to extend this period.
See also our previous package on Brexit.