Brexit: What lies ahead?

Article 50 of the Treaty of Lisbon gives any existing member of the European Union the right to quit unilaterally and outlines the procedure for doing so. It gives the state concerned two years to negotiate a deal for its exit. Once Article 50 is triggered, it cannot be stopped, except by the unanimous consent of all member states.

There are five elements in Article 50. They state that the exiting country must notify the European Council formally and that it is given a two-year period to reach an agreement. Article 50 also states that the country concerned cannot take part in EU’s internal discussions about its departure. The exit deal must be approved by a "qualified majority" and must also get the backing of MEPs.

The UK decided in favor of leaving the EU in a referendum in June 2016. After lengthy parliamentary debates and legal procedures, Prime Minister Theresa May looks all set to send a formal letter of notification to EU President Donald Tusk on March 29, which will trigger Article 50. Officials in Brussels have already outlined a divorce bill for Britain of between 55 and 60 billion euros.

The EU is expected to give a first response later this week. A summit of EU leaders on April 29 is then to lay down guidelines for the Brexit talks, which are expected to start in May or June. The hardest part of the negotiations will be determining the status of more than a million Britons living in other parts of the EU and of some 3 million EU citizens in Britain, and working out trade details.

By autumn this year, the UK government is expected to introduce legislation to leave the EU and put all existing EU laws into British law - the Great Repeal Bill. The possible move will annul the 1972 European Communities Act (ECA), which gives EU law instant effect in the UK, and give parliament the power to absorb parts of EU legislation into UK law, scrapping elements it does not want to keep.

EU leaders have said they want to conclude the talks within 18 months to allow the terms of the exit to be ratified by UK and the European Parliaments as well as the EU states. If no agreement is reached in two years and no extension is agreed, the UK automatically leaves the EU and all existing agreements.

The fifth paragraph of Article 50 raises the possibility of a state wanting to rejoin the EU after having left it. This would be considered under Article 49.