British Prime Minister Theresa May has said she will trigger Article 50 of the Lisbon Treaty by the end of this month, formally beginning Brexit negotiations. What exactly is the Lisbon Treaty and what does Article 50 stipulate?

The Lisbon Treaty is an international agreement which amends the two treaties that form the constitutional basis of the EU. It is designed to enhance the efficiency and democratic legitimacy of the union. The treaty was signed in December 2007, and 27 EU members were present. The UK prime minister at the time, Gordon Brown, did not take part in the main ceremony, but signed some hours later.

The treaty's Article 50 gives a member state the right to withdraw from the union, in accordance with its own constitutional requirements. Any member state that decides to withdraw should notify the EU Council of its intention. 

The union would then negotiate and conclude an exit agreement with the state. The negotiations would be conducted on behalf of the union by the council, after obtaining the consent of the EU parliament. 

The state would no longer be bound by the treaty once the withdrawal agreement is executed, or if no deal is reached after two years of negotiations. There must be a unanimous agreement if the negotiating period is to be extended. 

During the negotiating period, the state must still adhere to EU law, but it cannot participate in official EU discussions regarding its own withdrawal.