LONDON (AP) — A timeline of key events related to Britain's decision to leave the European Union:
Jan. 23, 2013: British Prime Minister David Cameron promises a referendum on Britain's membership to the EU if the Conservative party is elected in the next general election. He does so to try to garner support among euro-skeptics within his own party.
Sept. 18, 2014: Scottish voters decide in a referendum to remain part of the United Kingdom rather than become an independent country.
May. 7, 2015: British voters elect a majority Conservative government. Cameron confirms in his victory speech that there will be an in/out referendum on European Union membership.
Feb. 20, 2016: Cameron announces that he has negotiated a deal with EU leaders which will give Britain "special status." He confirms that he will campaign for Britain to remain in the 28-nation bloc. The referendum date is set for June.
Feb. 21: Cameron is struck with a severe blow as one of his closest Conservative allies, the media-savvy Boris Johnson, joins the Leave campaign.
June 16: One week before the referendum, Labor MP and Remain campaigner Jo Cox is killed by extremist Thomas Mair who shouted "Britain First" before killing the mother of two.
June 23: Britain votes 52 percent to 48 percent to leave the European Union. As the results come in, UKIP leader Nigel Farage proclaims that this day should be considered Britain's "independence day."
June 24: Cameron says he will resign in light of the results because Britain needs "fresh leadership" to take the country in a new direction.
July 11: Following a heated leadership contest, Home Secretary Theresa May becomes prime minister-elect when her competitors withdraw from the race.
Oct. 2: May says that Britain will begin the formal process of leaving the EU by the end of March 2017. In order to do this the British government would have to invoke Article 50 of the EU's Lisbon Treaty.
Jan. 24, 2017: The British Supreme Court rules that parliamentary approval is needed before Article 50 can be triggered by government.
March 13: Britain's Parliament approves a bill giving the government the authority to invoke Article 50. Scotland's first minister, Nicola Sturgeon, says she plans to have a second referendum on Scottish independence in late 2018 or early 2019.
March 28: May signs the letter that will trigger Brexit at 10 Downing Street.
March 29: Brexit will be formally triggered when the letter is hand-delivered to EU Council President Donald Tusk at around 1130 GMT (7:30 a.m. EDT).