London (CNN) UK Prime Minister Theresa May will begin the formal process of ending Britain's 44-year membership of the European Union Wednesday, kickstarting a two-year process that will end in Brexit.

May signed a formal letter to the European Union in Downing Street on Tuesday night, declaring that Britain is triggering Article 50 of the Lisbon Treaty, effectively the EU's rulebook.

The letter will be hand-delivered to the President of the European Council in Brussels at 1.30 p.m. Wednesday (7.30 a.m ET).

British Prime Minister Theresa May signs the official letter to European Council President Donald Tusk invoking Article 50.

On Wednesday, at the same time across the channel in London, May will stand to deliver a statement to the House of Commons, confirming that the Brexit process has begun.

Before heading to the Commons, May chaired a meeting of her Cabinet in 10 Downing Street. "It's a great day," said Foreign Secretary Boris Johnson as he left the meeting.