The UK voted to leave the EU in a referendum held on 23 June 2016. In the aftermath of the vote, Northern Ireland's First Minister and deputy First Minister on 10 August 2016 wrote to the newly appointed UK Prime Minister Theresa May to set out an "initial assessment" of issues "of particular significance" to Northern Ireland. They stated their shared objective that the land border "will not become an impediment to the movement of people, goods and services" on the island of Ireland. The Prime Minister's response on 14 October 2016 described the "future of the border with Ireland" as an "important priority for the UK" and stated the government's "wish to see the continuance of the free movement of people and goods" across it. 

On 17 January 2017 Prime Minister May set out the UK's 12 negotiating objectives for exiting the EU in a speech at Lancaster House. Several weeks later, Prime Minister May sent a letter to European Council President Donald Tusk giving formal notification of the UK's intention to leave the EU thereby triggering the withdrawal process laid down in Article 50 of the Treaty on the European Union (TEU).

Following UK notification, EU leaders in the European Council adopted Guidelines Following the United Kingdom's Notification under Article 50 TEU on 29 April 2017. UK-EU negotiations on the terms of withdrawal began in June 2017. According to an agreed timetable, negotiations on future UK-EU relations would not start until 'sufficient progress' had been made on withdrawal issues, namely: citizens rights, the financial settlement and arrangements for the unique circumstance on the island of Ireland. Early on in negotiations, the UK government published Northern Ireland and Ireland: position paper which set out UK objectives in some more detail. On 22 September 2017, the UK Prime Minister set out UK vision for a "deep and special partnership" between the UK and EU after Brexit in a speech at Florence.  

In December 2017, UK and EU negotiators produced a Joint Report on progress in the negotiations. This contains key commitments concerning the Northern Ireland dimension to withdrawal (see especially paragraphs 49 and 50). The agreements and commitments in the Joint Report were translated into a first draft Withdrawal Agreement by the European Commission in early 2018 and shared with the UK. A further draft indicating levels of agreement between the UK and the EU was published in March 2018. This concerned the terms of the UK's withdrawal. As for the post-withdrawal relationship between the UK and the EU, the European Council adopted Guidelines on 23 March 2018.

A further report on the state of the withdrawal negotiations - this time in the form of a joint statement - was issued on 19 June 2018. It identified areas where agreement was still outstanding. It was not until November 2018 that negotiations were concluded and the text of a draft Withdrawal Agreement agreed. Both the UK government and the European Commission issued "explainers" and/or slides of what had been agreed. The UK Prime Minister, Theresa May, was unable to secure parliamentary support for what had been agreed with the EU. She resigned and was replaced by Boris Johnson who immediately sought to re-open what had been agreed, particularly regarding the so-called 'backstop' Protocol on Ireland/Northern Ireland. He set out the new UK government's position in a letter to the President of the European Council, Donald Tusk, on 19 August 2019.

The Protocol was then revised, and agreement reached for a second time on the terms of the UK's withdrawal from the EU.  Again, the UK and the EU issued explainers. The Withdrawal Agreement was then approved by the UK Parliament and the European Parliament and signed. The EU published its official version of the Withdrawal Agreement - including the Protocol - on 31 January 2020. UK membership of the EU ceased at 2300 UK time - midnight in Brussels.