The UK is due to leave the European Union (EU) on 31 January 2020, after Prime Minister Boris Johnson's Brexit deal was passed by MPs. For those not following every twist and turn, this guide covers the basics. What is the European Union? The EU is an economic and political union involving 28 European countries. It allows free trade and free movement of people, to live and work in whichever country they choose. The UK joined in 1973 (when it was known as the European Economic Community). If the UK leaves, it would be the first member state to withdraw from the EU. Why is the UK leaving? A public vote - or referendum - was held on Thursday 23 June 2016, to decide whether the UK should leave or remain. Leave won by 52% to 48%. The referendum turnout was very high at 72%, with more than 30 million people voting - 17.4 million people opting for Brexit. Why hasn't Brexit happened yet? Brexit was originally due to happen on 29 March 2019. That was two years after then Prime Minister Theresa May triggered Article 50 - the formal process to leave - and kicked off negotiations. Under Mrs May, the deadline was delayed twice after MPs rejected her Brexit deal - eventually pushing it to 31 October 2019. Despite negotiating a revised deal, Mr Johnson missed the latest deadline, after MPs failed to pass it into law. The EU then agreed to a further extension until 31 January 2020. Theresa May making her final speech outside Theresa May resigned after her Brexit deal was rejected three times Why did Parliament reject Theresa May's Brexit deal? The main sticking point for many Conservative MPs and the DUP (the government's ally in Parliament at the time) was the Irish backstop. The backstop was designed to ensure there would be no border posts or barriers between Northern Ireland and the Republic of Ireland after Brexit.