Prime Minister Theresa May invokes Article 50 of the Treaty on European Union, formally beginning the United Kingdom's withdrawal from the European Union.

The prime minister of the United Kingdom is the head of government of the United Kingdom. The prime minister advises the sovereign on the exercise of much of the royal prerogative, chairs the Cabinet, and selects its ministers. Modern prime ministers hold office by virtue of their ability to command the confidence of the House of Commons, so they are invariably members of Parliament.

Theresa Mary May, Baroness May of Maidenhead, is a British politician who served as Prime Minister of the United Kingdom and Leader of the Conservative Party from 2016 to 2019. She previously served as Home Secretary from 2010 to 2016. She was Member of Parliament (MP) for Maidenhead from 1997 to 2024, and has been a member of the House of Lords since August 2024. May was the second female British prime minister, after Margaret Thatcher, and the first woman to have held two of the Great Offices of State. May is a one-nation conservative.

On 29 March 2017, the United Kingdom (UK) invoked Article 50 of the Treaty on European Union (TEU) which began the member state's withdrawal, commonly known as Brexit, from the European Union (EU). In compliance with the TEU, the UK gave formal notice to the European Council of its intention to withdraw from the EU to allow withdrawal negotiations to begin.

The Treaty on the European Union (2007) is one of the primary Treaties of the European Union, alongside the Treaty on the Functioning of the European Union (TFEU). The TEU forms the basis of EU law, by setting out general principles of the EU's purpose, the governance of its central institutions, as well as the rules on external, foreign and security policy.