The Conservative Party pledged an in-out referendum on membership of the European Union after a renegotiation. The referendum took place on 23 June 2016, and resulted in a vote to leave the European Union – a policy commonly referred to as Brexit. Prime Minister Theresa May signed the notice under Article 50 of the Lisbon Treaty which officially began Britain's withdrawal from the EU, on 28 March 2017, and at 12:20 on 29 March 2017, the UK ambassador Tim Barrow delivered the notice to EU president Donald Tusk, officially triggering what was intended to be a two-year process of leaving the European Union.