Liz Truss became the shortest-serving prime minister in history on October 20 after only 44 days in office.
The Tory leader announced her resignation after weeks of turmoil in Downing Street and furious backbench MPs expressing no confidence in her leadership.
In a statement of just 88 seconds, Ms Truss admitted she was unable to deliver on her mandate from the summer's Conservative Party leadership election.
In the statement, she said will stay on in No 10 until a successor is chosen via a leadership election.
But when will the winner of the leadership contest be announced and why has Boris Johnson dropped out of the race?
Here's everything you need to know.
When will the new prime minister be announced?
Tory leadership hopefuls, which currently include Rishi Sunak and Penny Mordaunt, have until 2pm today to win at least 100 nominations from MPs to get on the ballot for Downing Street.
As there are 357 Tory MPs, a maximum of three possible leaders will be able to enter the race before 2pm today.
If only one candidate secures enough nominations, they are declared the winner and will become our next prime minister.
As it stands, Mr Sunak is certain to be on the ballot and may even be coronated, after getting around 150 public backers, while Ms Mordaunt's position is uncertain as she was hovering on around 25 public backers last night.
Elsewhere, former PM Boris Johnson pulled out of the leadership race last night in a chaotic plot twist.
If both Mr Sunak and Ms Mordaunt get 100 nominations before 2pm, the Tory ballot will take place.
The ballot will be held at 3.30pm on Monday with the result at 6pm. The candidate with the fewest MP supporters will then be knocked out of the leadership race.
Why did Boris Johnson pull out?
The former PM announced that he was pulling out of the Tory leadership race after admitting he would be too divisive as Tory grandees voiced fury and his campaign flopped.
In a 9pm statement last night (October 23), he claimed without evidence that he'd reached 102 backers - but confessed: “This is simply not the right time.”
Prior to the announcement, Johnson had jetted home from his Dominican Republic beach holiday to drum up support despite a chorus of Tories warning his comeback would destroy the party and the country.
But hours after backers claimed he had the 100 MPs he needed to get on the ballot, Mr Johnson declared he would not be standing after all.
The MP boasted that he had led the Tories into a "massive election victory" and was "uniquely placed to avert a general election now" by fulfilling the 2019 manifesto.
He added: "I have sadly come to the conclusion that this would simply not be the right thing to do. You can’t govern effectively unless you have a united party in parliament."
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