In compliance with the Treaty on European Union (TEU), the United Kingdom on this day in 2017 gave formal notice to the European Council, of its intention to withdraw from the European Union (EU,) allowing withdrawal negotiations to begin. The process of leaving the EU was initiated by a referendum held in June 2016 which resulted in 52% voting in favour of British withdrawal. In October 2016, the British prime minister, Theresa May, announced that Article 50 would be invoked by “the first quarter of 2017”. On January 24, 2017 the Supreme Court ruled that the process could not be initiated without an authorizing Act of Parliament. Consequently, the European Union (Notification of Withdrawal) Act 2017 empowering the prime minister to invoke Article 50 was enacted in March 2017.
Invocation of Article 50 occurred on this day in 2017, when Tim Barrow, the Permanent Representative of the United Kingdom to the European Union, formally delivered by hand a letter signed by the prime minister to Donald Tusk, the president of the European Council in Brussels. The letter also contained the United Kingdom’s intention to withdraw from the European Atomic Energy Community (EAEC or Euratom). This meant that the UK was scheduled to cease being a member of the EU at the end of March 29, 2019 Brussels time (UTC+1), which would be 23:00 on March 29, British time. This was extended by two weeks to give the Parliament of the United Kingdom time to reconsider its rejection of the agreement on withdrawal conditions, particularly in the House of Commons. The UK was due to leave the EU at the end of April 12, 2019 (24:00 Central European summer time; 23:00 British summer time), however a further ‘flexible’ extension was granted until October 31, 2019 after talks at the European Council meeting on April 10. After another extension in October 2019 and subsequent negotiations, a withdrawal agreement was negotiated in late October 2019 and ratified by both parties in January 2020: consequently the UK left the EU at 23:00 on January 31, 2020 and entered the transition period.
-Wikipedia