Supporters, who'd gathered expecting to hear Boris Johnson formally announce his candidacy, were stunned - his announcement met with audible gasps.
It's the biggest political surprise since British Prime Minister David Cameron quit after losing the referendum on remaining in the bloc.
"But I must tell you, my friends, you who have waited faithfully for the punchline of this speech, that having consulted colleagues, and in view of the circumstances in parliament, I have concluded that person cannot be me."
Reaction to Mr Johnson's announcement has been swift.
Conservative MP Andrew Mitchell is calling for the next leader to be a Leave campaign supporter.
"It's been a somewhat unusual day for the Conservative Party but the issue, of course, is who is going to stand for the progressive, one-nation policies which Boris believes in, and you know, for people like me, I think the next leader probably has to be a Brexiteer. It's not impossible for a non-Brexiteer to become Prime Minister and leader of the party but if you look forward, there are very difficult circumstances in doing a deal with Europe."
Others were more scathing, with Conservative MP Nigel Evans saying Mr Johnson's announcement makes "'House of Cards' look like 'Teletubbies.'"
A visibly angry Lord Michael Heseltine, the man who once challenged Margaret Thatcher for Tory leadership, handed down this brutal verdict.
"He's ripped the party apart. He's created the greatest constitutional crisis of modern times. He's knocked billions off the value of the nation's savings. He's like a general that led his army to the sound of guns and at the sight of the battlefield, abandoned the field to the claims of his adjutant who says he wasn't up to the job in the first place. I have never seen so contemptible and irresponsible a situation."
The adjutant Lord Heseltine refers to is Boris Johnson's Brexit campaign ally, Justice Secretary Michael Gove, who suddenly withdrew his backing for Mr Johnson while at the same time throwing his own hat into the ring.
"Boris is an amazing and an impressive person but I've realised, in the last few days, that Boris isn't capable of building that team and providing that unity and so I came reluctantly but firmly to the conclusion that, as someone who argued from the beginning that we should leave the European Union, and as someone who wanted to ensure that a bold, positive vision for our future was implemented, that I had to stand for the leadership of the Conservative Party."
Boris Johnson's withdrawal makes Theresa May the new favourite to succeed David Cameron.
Despite backing the Remain campaign she announced her own candidacy, promising to deliver the EU withdrawal British voters have demanded.
"Brexit means Brexit. The campaign was fought, the vote was held, turnout was high and the public gave their verdict. There must be no attempts to remain inside the EU, no attempts to rejoin it through the back door and no second referendum. The country voted to leave the European Union and it's the duty of the government and parliament to make sure we do just that."
She's offering party colleagues stability, experience and competence.
"I know I'm not a showy politician. I don't tour the television studios. I don't gossip about people over lunch. I don't go drinking in parliaments bars. I don't often wear my heart on my sleeve. I just get on with the job in front of me."
Conservative Party MPs will narrow a field of five leadership candidates down to two, and party members will then vote on which of them will become party leader and presumptive prime minister.
In addition to Therese May and Michael Gove, the candidates are Stephen Crabb, the cabinet minister responsible for pensions, Liam Fox, a right-wing former defence secretary, and Andrea Leadsom, a minister in the energy department.
The party says it's aiming to replace its leader by September 9th.
Britain's new prime minister faces a huge task to unite the party and the country, and persuade the EU to offer some kind of deal -
balancing voter calls to reduce immigration with business hopes of maintaining access to EU markets.
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