Theresa May will step down as leader of the Conservative Party today.
The Prime Minister announced in a tearful speech two weeks ago that she would be leaving on June 7.
She told the waiting press outside Downing Street that her resignation was in the "best interests of the country", adding: "I have done my best".
However, she will remain acting leader while a leadership contest takes place.
In the wake of her announcement, the hastag #Mexit began trending on Twitter, a blend of 'May' and 'exit', and a nod to 'Brexit', which underpinned her premiership.
The new Prime Minister is expected to be appointed in the week beginning July 22.
There are currently 11 MPs vying to replace Mrs May, after two contenders dropped out following a rule change by the 1922 Committee.
Under the new contest rules, candidates will undergo at least a dozen hustings, and after a series of MP ballots, the final two competitors will go into a run-off decided by the party's 160,000-strong membership.
But Labour leader Jeremy Corbyn said whoever became the next leader should call a General Election immediately.
"Whoever becomes the new Tory leader must let the people decide our country’s future, through an immediate General Election," he wrote on Twitter.
Currently, Boris Johnson is the bookies' favourite to get the job, and has the largest number of MPs publicly supporting him.
Michael Gove is close behind Mr Johnson, with Ladbrokes placing his odds of becoming the next party leader at 5/1.