On 23 June 2016, the UK electorate voted to leave the European Union and on 29 March 2017, the UK notified the European Council of its intention to withdraw from the EU in accordance with Article 50 of the Treaty on European Union. The European Council adopted a set of political guidelines on 29 April 2017, which define the framework for the negotiations and set out the EU's overall positions and principles. Brexit negotiations between the EU and the UK began on 19 June 2017.

On 14 November 2018 the Heads of State/Government of the remaining 27 EU Member States approved a Withdrawal Agreement negotiated by the European Commission and the UK Government. A Political Declaration on future EU-UK relations was also approved at the same time.

The UK’s membership of the EU was initially due to lapse on 30 March 2019, two years from the day it formally notified of its intention to withdraw from the EU. However, the European Council decided, in agreement with the UK, to extend the two-year period provided for by Article 50, until 31 October 2019.

On 17 October 2019, it was announced that the EU and the UK had reached a revised Withdrawal Agreement, which includes a revised Ireland/Northern Ireland Protocol and a revised Political Declaration. Under the revised Withdrawal Agreement, Northern Ireland would remain aligned to certain Single Market rules, avoiding the requirement for a customs border on the island of Ireland, while ensuring that Northern Ireland remains part of the UK's customs territory.

On 28 October 2019, the EU agreed a further three-month extension to the Brexit deadline of 31 October 2019, making the new Brexit deadline 31 January 2020. On 24 January 2020, President of the EU Council, Charles Michel and the President of the European Commission, Ursula von der Leyen both formally signed the Brexit Withdrawal Agreement.  This followed on from the legislation implementing the agreement receiving royal assent on 23 January 2020.  On 24 January 2020, UK Prime Minister, The Rt. Hon. Boris Johnson MP also signed the Withdrawal Agreement cementing the UK’s departure from the EU on 31 January 2020.  The Withdrawal Agreement was presented to the European Parliament on 29 January for ratification, which ensured the orderly exit of the UK from the EU at midnight on 31 January 2020.

As the Withdrawal Agreement became effective, a transition period began on 1 February 2020. The UK is no longer represented in the EU institutions, agencies, bodies and offices but EU law continued to apply in the UK until the end of the transition period on 31 December 2020. This meant the UK continued, for the purposes of the movement of goods, as if it were a full EU Member State until 31 December 2020.
Part 15 of the Withdrawal of the United Kingdom from the European Union (Consequential Provisions) Act 2019 came into operation with effect from 31 January 2020 at 11pm.  This means that where the term ‘Member State’ is used in any enactment it should be interpreted as including the UK for the duration of the transition period created by the Withdrawal Agreement.

On 25 February 2020, the European Council authorised the opening of the future partnership negotiations with the UK. The negotiating directives adopted were based on existing European Council guidelines and conclusions, as well as the Political Declaration agreed between the EU and the UK in October 2019. On 18 March, the European Commission published a draft legal agreement for the future EU-UK partnership. This translated into a legal text, the negotiating directives approved by Member States in the General Affairs Council on 25 February 2020, in line with the Political Declaration agreed between the EU and the UK in October 2019.

On 20 May 2020, the UK government published its plans for how the Northern Ireland protocol of the Withdrawal Agreement would work.

The UK government confirmed it would not consider an extension of the Brexit transition period at the second meeting of the EU-UK Joint Committee under the Withdrawal Agreement on 12 June 2020 (i.e. the option to extend had been provided for under the terms of the Withdrawal Agreement).  During the meeting, both the EU and UK resolved to accelerate their work to implement the Withdrawal Agreement and agreed that the Joint Committee should meet again in early September 2020.

On 9 September 2020, the Irish Government published its Brexit Readiness Action Plan to provide clear and concise advice on the steps businesses and individuals needed to take in order to prepare for the end of the Brexit transition period on 31 December 2020. The Government also published the General Scheme of the 2020 Brexit Omnibus Bill on the same day, which addressed a wide range of complex issues that would arise at the end of the transition period.

The UK government published the draft United Kingdom Internal Market Bill on 9 September 2020 which, if adopted as proposed, would be in clear breach of substantive provisions of the Ireland/Northern Ireland Protocol: Article 5 (3) & (4) and Article 10 on custom legislation and State aid, including, the direct effect of the Withdrawal Agreement (Article 4). In addition, the UK government would be in violation of the good faith obligation under the Withdrawal Agreement (Article 5) as the draft Bill jeopardised the attainment of the objectives of the Agreement.

Vice-President of the EU Maroš Šefčovič called for an extraordinary meeting of the EU-UK Joint Committee on Thursday, 10 September 2020 to request the UK government to elaborate on its intentions and to respond to the serious concerns of the EU. A meeting took place in London between Vice-President Maroš Šefčovič and The Rt. Hon. Michael Gove MP, Chancellor of the Duchy of Lancaster. Following the meeting, the Vice-President stated, that the timely and full implementation of the Withdrawal Agreement, including the Protocol on Ireland/Northern Ireland, was a legal obligation. The EU expected the letter and spirit of this Agreement to be fully respected, as violating the terms of the Withdrawal Agreement would break international law and put at risk the ongoing future relationship negotiations.

On 7 December 2020, the Co-Chairs of the EU-UK Joint Committee, European Commission Vice-President Maroš Šefčovič and the UK Chancellor of the Duchy of Lancaster, The Rt Hon Michael Gove MP, met to address the outstanding issues relating to the implementation of the Withdrawal Agreement. In a joint statement on Tuesday, 8 December 2020 the two Co-Chairs confirmed that an agreement in principle had been found on Border Control Posts/Entry Points, specifically for checks on animals, plants and derived products, export declarations, the supply of medicines, the supply of chilled meats, and other food products to supermarkets, and a clarification on the application of State aid under the terms of the Protocol.

The parties also reached an agreement in principle with respect to the decisions the Joint Committee had to take before 1 January 2021 concerning the practical arrangements regarding the EU's presence in Northern Ireland.

Following the agreement in principle, the EU-UK Joint Committee met virtually on 17 December 2020 to endorse all formal decisions and other practical solutions related to the implementation of the Withdrawal Agreement, as of 1 January 2021.

The Withdrawal Agreement, and the Protocol on Ireland and Northern Ireland in particular, was implemented on 1 January 2021. In view of the mutually agreed solutions, the UK agreed to withdraw clauses 44, 45 and 47 of the UK Internal Market Bill, and not to introduce any similar provisions in the Taxation Bill.

The Withdrawal of the United Kingdom from the European Union (Consequential Provisions) Act 2020 was signed into law by the President of Ireland on 10 December 2020.

On 24 December 2020, the EU signed the EU-UK Trade and Cooperation Agreement. The Agreement concluded between the EU and the UK sets out preferential arrangements in areas such as trade in goods and in services, digital trade, intellectual property, public procurement, aviation and road transport, energy, fisheries, social security coordination, law enforcement and judicial cooperation in criminal matters, thematic cooperation and participation in Union programmes. It is underpinned by provisions ensuring a level playing field and respect for fundamental rights.

Regarding entry into application of the Trade and Cooperation Agreement, the European Commission will apply it on a provisional basis, for a limited period of time until 28 February 2021. The Commission will propose Council decisions on the signature and provisional application, and on the conclusion of the Agreement. The European Council, acting by the unanimity of all 27 Member States, must adopt a decision authorising the signature of the Agreement and its provisional application as of 1 January 2021. Once this process is concluded, the Trade and Cooperation Agreement between the EU and the UK can be formally signed.

The European Parliament will then be asked to give its consent to the Agreement. As a final step on the EU side, the European Council must adopt the decision on the conclusion of the Agreement.