The scope of the negotiations
On 23 June 2016 the people of the United Kingdom voted on the question: "Should the United Kingdom remain a member of the European Union or leave the European Union?" The result of the referendum was that 17.4 million people (51.9% of the total) voted to leave the EU, and 16.1 million (48.1%) voted to remain.
Under Article 50 of the Treaty on European Union, any State withdrawing from the EU must formally notify the European Council of its intention. The UK did so by means of the Prime Minister's letter to Donald Tusk, President of the European Council, dated 29 March 2017.
Article 50 also provides for a two-year period following notification, within which the two sides are to reach agreement on the withdrawal arrangements, while taking account of the framework for their future relationship. The process is described in more detail in the House of Lords European Union Committee's May 2016 report on The process of withdrawing from the European Union.
Who conducts the negotiations?
The two sides to the Brexit negotiations are the UK, as the withdrawing State, and the EU and its 27 remaining Member States (the 'EU 27').
The EU is represented by the European Commission, which has appointed Michel Barnier, a former French MP, Minister, MEP and European Commissioner, as its Chief Negotiator; the UK by the Department for Exiting the European Union (DExEU), led by the Secretary of State, Rt Hon David Davis MP.
Progress so far
On 19 June 2017 the UK Government and the European Commission jointly published the terms of reference for the Article 50 negotiations.
The negotiations were split into two broad 'phases'. Phase 1 focused on withdrawal issues, including citizens' rights, the financial settlement, and the impact of Brexit on the island of Ireland. The two sides agreed that only once 'sufficient progress' had been reached on phase 1, would negotiations move on to phase 2, focusing initially on transition, and then on the framework for future relations between the UK and the EU.
The negotiating rounds in phase 1 consisted of plenary sessions and negotiating group meetings. In the latter, working groups of officials discussed specific issues and reported back to the plenary sessions, which were co-chaired by the "Principals", Michel Barnier and Rt Hon David Davis MP, or by the "Coordinators": Sabine Weyand for the Commission, and Oliver Robbins for the UK.
On 8 December 2017, following intense negotiations, the European Commission recommended to the European Council that 'sufficient progress' had been made in phase 1 of the negotiations. The European Council, at its meeting on 15 December 2017, agreed with the Commission, and on that basis adopted guidelines to move to the next phase of negotiations.
It is not yet clear how negotiations on 'phase 2' issues will be organised, but on 29 January 2018, the Council adopted supplementary directives for the Brexit negotiations, which establish the EU 27 position regarding a transition period. These directives provide the Commission, as the EU negotiator, with a mandate to start discussions with the UK.
The role of the other key EU institutions
Under Article 50 of the Treaty on European Union, the EU 27 are authorised to meet without the UK to discuss matters relating to the UK's withdrawal from the EU. The European Council, made up of the Heads of State or Government of the EU 27, is responsible for setting the EU's guidelines for the Brexit negotiations.
The European Council has established a working group to work on the UK's exit from the EU, headed by Belgian diplomat Didier Seeuws. It liaises closely with the European Commission and ensures that the European Council’s priorities are respected.
The European Parliament must give its consent to any draft withdrawal agreement agreed between the EU and the UK. All Members of the European Parliament (including UK MEPs) will be entitled to vote, and consent will require a simple majority.
Pending this final stage of the process, the European Parliament agreed resolutions on 5 April, 3 October, and 13 December 2017, setting out its priorities for the negotiations.
In addition, the European Parliament has appointed the former Prime Minister of Belgium, Guy Verhofstadt MEP, as its Brexit Coordinator, also chairing a 'Brexit Steering Group' (BSG). The BSG, composed of the leaders of political groups within the Parliament, is tasked with coordinating and preparing the EP's debates and resolutions on the UK withdrawal. Only those political groups that supported the European Parliament's resolutions on Brexit are represented on the BSG, meaning that the political groups to which UK Conservative and UKIP MEPs belong are not represented.
Negotiating texts and other documents
For both parties the default is transparency. Texts that are intended for discussion at any negotiating round should be shared at least one week in advance wherever possible. The European Commission and the UK Government have also both set up web pages relating to the negotiations, on which relevant documents are published.