THERESA May is on the verge of facing a leadership challenge amid open Tory mutiny over her draft Brexit deal.
A raft of Conservative MPs are expected to follow chief hardline Brexiteer Jacob Rees-Mogg in submitting letters of no confidence against their party leader.
But Leave-backing Tories would need to meet a threshold of 48 letters being submitted to the 1922 Committee to trigger a confidence vote.
Mrs May, who intends to contest any such vote, would need the support 158 Tory MPs to see off a leadership challenge – keeping her position safe for another 12 months.
The backlash has fuelled fears of the UK crashing out of the European Union with no deal – and the return of a hard border in Ireland.
It was unclear on Thursday night whether Tories unhappy with the British prime minister's deal on the UK's withdrawal from the EU had secured enough no-confidence letters.
The move against Mrs May came as she was hit by a series of ministerial resignations on Thursday in one of the toughest days of her premiership.
In sensational developments, Dominic Raab walked out as Brexit secretary and Esther McVey also left her job in the cabinet as work and pensions secretary.
Two more junior ministers – Suella Braverman at the Brexit department and Shailesh Vara at Northern Ireland – also quit.
And Mrs May spent nearly three hours fielding largely critical questions from MPs about her Brexit plans.
Her UK-EU deal includes a 'backstop' arrangement, aimed at preventing a hard border between Northern Ireland and the Republic if this is not otherwise resolved through a future UK-EU trade deal.
It would effectively mean any customs and regulatory checks would be at the Irish Sea border – but many unionists and Brexiteers see this as separating Northern Ireland from Britain.
In the House of Commons, Mrs May faced Tory backbench accusations that the Brexit deal agreed by cabinet on Wednesday was "dead on arrival" and would never survive the parliamentary vote expected next month.
Only a handful of her own MPs spoke up in favour of the plan, which was denounced by Labour leader Jeremy Corbyn as "half-baked".
But in a defiant press conference in 10 Downing Street, the prime minister insisted she would "see this through".
Standing before a pair of union flags, Mrs May compared herself to her stubborn but effective cricketing hero as she told reporters: "What do you know about Geoffrey Boycott? Geoffrey Boycott stuck to it and he got the runs in the end."
It followed a chaotic day in which the value of the pound plunged amid widespread doubts over whether Mrs May could deliver her deal – or even cling on to power.
Mr Raab, who only in July replaced David Davis as Brexit secretary – quit the job, warning the deal represented a "very real threat to the integrity of the United Kingdom" because of provisions for Northern Ireland.
And a resigning Ms McVey told the prime minister she could not defend the agreement approved by cabinet in a stormy five-hour meeting.
Mr Rees-Mogg, who chairs the influential European Research Group of Eurosceptic Tory MPs, dramatically announced that he had submitted a letter of no confidence in Mrs May's leadership, declaring that her deal "has turned out to be worse than anticipated and fails to meet the promises given to the nation by the prime minister".
In her press conference, Mrs May said: "I believe with every fibre of my being that the course I have set out is the right one for our country and all our people."
She added: "Leadership is about taking the right decisions, not the easy ones.
"As PM my job is to bring back a deal that delivers on the vote of the British people, that does that by ending free movement ... ensuring we are not sending vast annual sums to the EU any longer, ending the jurisdiction of the European Court of Justice, but also protects jobs and protects people's livelihoods, protects our security, protects the union of the United Kingdom.
"I believe this is a deal which does deliver that, which is in the national interest, and am I going to see this through? Yes."
