May will resign before next election
At least 175 Conservative Party politicians have indicated support for British Prime Minister Theresa May as they head into a confidence vote in her leadership, based on public statements in the media and social media.
Mrs May needs to secure a simple majority - 158 of 315 politicians - to remain leader. A secret ballot of MPs has started.
She seems to have at least 175 MPs on her side but British political commentators report some politicians who back Mrs May in public say privately they will vote against her.
In a dramatic development Mrs May said she would stand down before the next election, after she has steered Brexit through.
"In my heart I would like to lead the party into the next election, but I accept that won't happen," she said.
Some ministers were reportedly in tears after the PM's announcement.
Mrs May refused to put an exact date on when she would stand down despite calls for her to go as soon as Britain formally leaves the EU.
In a defiant press conference outside 10 Downing Street, Mrs May said changing Conservative leaders would "put our country's future at risk and create uncertainty when we can least afford it" and could lead to Brexit being delayed or prevented.
"Weeks spent tearing ourselves apart will only create more division," the PM said, warning that leadership challenge threatens to hand power to Labour.
"I stand ready to finish the job."
The PM said any new leader would have to extend the March 29 deadline for Britain's exit from the European Union.
"A new leader wouldn't be in place by the January 21 legal deadline, so a leadership election risks handing control of the Brexit negotiations to opposition MPs in Parliament," May said.
"A new leader wouldn't have time to renegotiate a withdrawal agreement and get the legislation through Parliament by March 29, so one of their first acts would have to be extending or rescinding Article 50, delaying or even stopping Brexit when people want us to get on with it."
Mrs May was due to fly to Dublin overnight to speak to Irish Taoiseach Leo Varadkar.
The besieged Prime Minister returned from a whirlwind tour of The Hague, Berlin and Brussels where she had been begging for further concessions on her Brexit deal to rush straight to Number 10 as Tory Brexiteers rounded up the required 48 letters to force a ballot that could end the PM's time as leader.
The man obliged to bring the vote forward, chair of the backbench 1922 committee Sir Graham Brady, requested a meeting with Mrs May.
He said the threshold had been "exceeded".
The front pages of all British newspapers claimed an imminent move was likely against Mrs May's leadership, and that Tory leadership aspirants were manoeuvring.
The former foreign secretary and leading Brexiteer Boris Johnson was positioning, as was the rising Tory star, Home Secretary Sajid Javid. Other names in the mix are Dominic Raab, Michael Gove and Jeremy Hunt, while David Davis, Amber Rudd and Andrea Leadsom were also mentioned.
Mrs May this week delayed until potentially January 21 the planned vote in the House of Commons on her deal, after it became obvious it was heading for a defeat by up to 200 votes.
Her frantic negotiations in Europe failed to break the impasse over the backstop - the agreement that the UK would remain within EU customs and other laws, if they could not strike a final deal within an agreed transition period.
UK MPs are refusing to accept the backstop because they won't be able to leave it without the agreement of the EU, arguing it ties them to the EU more firmly than current arrangements.
Mrs May needs only a simple majority of her 316 MPs to win the party vote, and is theoretically safe from challenge for 12 months if she does win.
But a very narrow victory would be so damaging to her already tattered authority Brexiteers are confident she would have to resign.
If she lost, there would then be a series of run-offs held between other candidates. The vote on the final two candidates goes to a postal ballot of grassroots members, meaning even a fast-tracked leadership content could take several weeks.
Ousted British prime ministers tend to stay on in their role for the period required to have a new prime minister chosen by the party and approved by the Queen.