Article 50 of the Treaty on European Union (the TEU) provides the legal basis and procedure for an EU Member State to withdraw from the Union. This route to withdrawal is the only one that complies with international law. Under Article 50, the Member State must provide notice of its intention to withdraw, which triggers the start of a two year period in which the Member State and the EU negotiate the terms of withdrawal, taking account of the framework for (rather than settling the terms of) their future relationship.
The UK gave notice of its intention to withdraw from the EU on 29 March 2017 (Article 50 notice), which has started a two year period in which the UK and the EU will negotiate the terms of withdrawal. The UK remains a Member State during this process. Read the letter to President Tusk here.
If no agreement on the arrangements for its withdrawal is reached, Brexit will occur automatically two years later on 29 March 2019.The UK Prime Minister has said that the default position, in the event no agreement is reached, would be that the UK would leave the EU on World Trade Organisation terms. The two year period can however be extended if negotiations are incomplete, but only with the unanimous consent of the Council of the EU (and thus all remaining 27 Member States). The two year period includes time at the end when the Council and Parliament consider the terms of the agreement, so the available period for negotiation is well under two years.
The Article 50 procedure has never before been used and the provision is somewhat opaque. This puts both the UK and the EU into unchartered waters. However, the EU has said that it will not consider negotiation of a post-exit arrangement, without finalizing negotiations on the terms of the UK’s withdrawal first.
The Council of the EU is ultimately responsible for concluding the withdrawal agreement with the UK on the basis of a qualified (weighted) majority, having sought the consent of the European Parliament.
It is not clear whether the UK could revoke the Article 50 notice, once given. Its irrevocability was assumed by both parties in the Miller litigation, but this interpretation is yet to be confirmed by the Court of Justice of the European Union (CJEU) or any other EU institution. An action before the Irish High Court to establish if Article 50 notice is or is not revocable once given has been discontinued.