Boris Johnson will not stand to be Conservative party leader.
After a morning of shock revelations, the former London major and champion of the campaign to leave the European Union, said was not the right person to lead the UK.
The move came after his Vote Leave colleague Michael Gove and home secretary Theresa May both announced they will run for leader.
Gove, the justice secretary, announced his surprise bid with a dig at Johnson who he said could not "provide the leadership or build the team for the task ahead". He had been expected to support Johnson's campaign after the pair were leading lights in bringing the UK out of the EU.
The shock announcement came as both Labour and Conservative parties headed for a summer in turmoil as a leadership race begins within both.
Home secretary Theresa May announced her leadership bid earlier on Thursday morning. She pitched herself as a leader to tackle "serious social reform" and said she would fight against "burning injustices".
Labour's former shadow business secretary Angela Eagle will also challenge Jeremy Corbyn for Labour leader on Thursday, according to the BBC.
The battles come after the UK voted to leave the European Union, a move which both parties campaigned against.
The Tory race is set to be between Gove, May, the work and pensions secretary Stephen Crabb, the ex-defence minister Liam Fox and Andrea Leadsom, all of whom have announced their bids. The contest was sparked when Prime Minister David Cameron announced he would resign by October.
Nicky Morgan, who had been rumoured to run, said she would back Michael Gove in his campaign to be the next Prime Minister.
In an article for the Spectator, Gove, who is a committed Christian, admitted he has previously said he did want to be Prime Minister.
He called for an "open and positive debate" about the UK's way forward and said he hoped to "provide unity and change".
The surprise challenge came after an email from Gove's wife revealed concerns about Johnson's leadership. In the email, which was leaked to Sky News, Sarah Vine urged her husband to seek assurances from Johnson he would receive a top spot before supporting him. Vine, a newspaper columnist, warned that without Gove's support the right-wing press would not support Johnson.
"Crucially, the membership will not have the necessary reassurance to back Boris, neither will [Daily Mail editor Paul] Dacre/[Rupert] Murdoch, who instinctively dislike Boris but trust your ability enough to support a Boris Gove ticket," she wrote.
The tensions in the Tory party are mirrored in the Labour party after Jeremy Corbyn refused to stand down despite loosing an overwhelming vote of no confidence from Labour MPs.
At a rally in London on Wednesday night, Corbyn agreed not everyone was with him but said he was determined to "carry on with his work".
Corbyn won the Labour leadership with an impressive groundswell of support from members last September but had failed to gain the loyalty of his MPs. A number of Labour-backing union leaders issued a joint statement to say Corbyn has a "resounding mandate" and that a leadership contest would be an "unnecessary distraction".
His shadow chancellor John McDonnell was one of 40 MPs to support Corbyn in a motion of no confidence earlier this week.
He called on Labour activists to show "absolute determination, courage but above all else solidarity" in the face of challenges to his leadership. "We're not going to be bullied by Labour MPs who refuse to accept democracy in our party".
Labour's deputy leader Tom Watson refused to stand a rival leader but said he had urged Corbyn to stand aside. But Eagle will mount a challenge on Thursday, according to the BBC, which will prompt the second leadership race in less than a year for Labour.
