Former London Mayor Boris Johnson, who had been considered one of the favourites to replace David Cameron as UK Prime Minister, said today he would not be standing.
The former London mayor's decision not to join the battle leaves Home Secretary Theresa May as hot favourite to be the next Prime Minister.
It came after the shock announcement by Johnson's fellow Brexit campaigner Michael Gove - who had widely been expected to be Mr Johnson's running mate - that he was putting himself forward for the leadership.
In a dramatic press conference just moments before the deadline for nominations passed, Mr Johnson said that the next Tory leader would have to unify his party and ensure that Britain stood tall in the world.
And he said: "Having consulted colleagues and in view of the circumstances in Parliament, I have concluded that person cannot be me."
The announcement came as Justice Secretary Michael Gove announced he would seek the Tory leadership after concluding that Boris Johnson "cannot provide the leadership or build the team for the task ahead".
The pro-Brexit campaigner who had been expected to back the ex-London mayor threw his hat in the ring.
"I have repeatedly said that I do not want to be prime minister. That has always been my view. But events since last Thursday have weighed heavily with me," he said.
"I respect and admire all the candidates running for the leadership. In particular, I wanted to help build a team behind Boris Johnson so that a politician who argued for leaving the European Union could lead us to a better future.
I have come, reluctantly, to the conclusion that Boris cannot provide the leadership or build the team for the task ahead
"But I have come, reluctantly, to the conclusion that Boris cannot provide the leadership or build the team for the task ahead.
"I have therefore decided to put my name forward for the leadership. I want there to be an open and positive debate about the path the country will now take. Whatever the verdict of that debate I will respect it. In the next few days I will lay out my plan for the United Kingdom which I hope can provide unity and change."
The declaration was a devastating blow for Mr Johnson, who had been expected to stand with the backing of the Justice Secretary.
During the EU referendum campaign, the two men worked side by side to secure the shock Brexit victory and were believed to have grown close.
But an email yesterday from Mr Gove's wife that was accidentally sent to the wrong person revealed behind the scenes concerns about the former mayor's leadership bid.
Daily Mail columnist Sarah Vine said in the email that her husband must secure a specific guarantee about his future before making any deal with Mr Johnson and should "not concede any ground".
Theresa May, who was in the Remain camp, has said the people's referendum decision has to be respected. If made prime minister, she said she would set up a unit to negotiate the exit, headed by somebody from the Leave campaign.
Theresa May.