On 29 March 2017, the UK government formally began the process of withdrawal by invoking Article 50 of the Treaty on European Union, with permission from Parliament. May called a snap general election in June 2017, which resulted in a Conservative minority government supported by the Democratic Unionist Party. UK–EU withdrawal negotiations began later that month. The UK negotiated to leave the EU customs union and single market. This resulted in the November 2018 withdrawal agreement, but the UK parliament voted against ratifying it three times. The Labour Party wanted any agreement to maintain a customs union, while many Conservatives opposed the agreement's financial settlement on the UK's share of EU financial obligations, as well as the "Irish backstop" designed to prevent border controls between Northern Ireland and the Republic of Ireland. The Liberal Democrats, Scottish National Party and others seek to reverse Brexit through a second referendum. Should a withdrawal agreement be passed, there will be a transition period until at least 31 December 2020, where the future relationship is to be negotiated.