Theresa May is on the brink of a leadership challenge after Tory MPs claimed the 48 no-confidence letters required for a contest have been submitted.
After a gruelling day of meetings with EU leaders and top Brussels officials in three European capitals, the prime minister returned to Downing Street just before 10pm on Tuesday.
Minutes earlier, the government chief whip Julian Smith had arrived at No 10 and is believed to have given her the bad news that she is close to facing a vote of confidence.
In highly unusual late-night developments, also seen in Downing Street was the Conservative Party chairman Brandon Lewis, who later left No 10 about 11pm – shortly before Mr Smith also departed.
Although there was no confirmation from the chairman of the backbench 1922 Committee Sir Graham Brady that he had received 48 letters, many Conservative MPs believe he has.
There were unconfirmed reports that Sir Graham had asked to see the prime minister either before or after Prime Minister's Questions, a session that will be extremely painful for Mrs May.
But other Conservative MPs believe the prime minister's enemies are exaggerating and over-hyping the scale of the mutiny and insist the 48-letter threshold has not been reached.
As well as PMQs, the prime minister also faces a potentially fraught meeting of her cabinet, at which ministers are expected to push her to step up preparations for a no-deal Brexit.
And if there is to be a no confidence vote, the format for the contest could be spelled out later by Sir Graham at the regular Wednesday meeting of the 1922 Committee of Tory backbenchers.
Amid a frenetic atmosphere at Westminster, many MPs expect Sir Graham to make a formal announcement confirming he has received the 48 letters and set in motion a contest that could begin within hours.
Until this week, Mrs May's supporters claimed she would see off a challenge to her leadership in which her opponents will need 159 votes – a majority of the Tories' 316 MPs – to topple her.
But after her controversial decision on Monday to call off the crunch vote on her Brexit deal, opinion against her among Conservative MPs has hardened and even some who were previously loyal to the PM have turned against her.
Claiming the 48-letter threshold has been reached, Brexiteer Tory MP Andrew Bridgen told Sky News: "I am aware of a number of colleagues who have put letters of no confidence in today.
"I believe that we have breached the 48-letter limit and that means if that's the case that Sir Graham Brady will be going to see the Prime Minister to call for a vote of confidence tomorrow."
A former minister told Sky News that more letters went in to Sir Graham in the hours before Mrs May returned to Downing Street, as MPs felt more confident that she would lose a confidence vote in her party.
The senior Conservative, who has not put in a letter yet but said he planned to when Mrs May returns from the EU summit at the end of this week if there is no progress on the Irish backstop issue, said: "Things have really shifted in the past week and I think she would lose that confidence vote now. This week is crucial.
"I and others have been holding back because of the difficulty of that confidence vote but the mood is shifting among MPs and our members, who were previously very supportive of the prime minister."
The latest Conservative MP to go public about writing a no confidence letter to Sir Graham was the former cabinet minister Owen Paterson, whose scathing letter was published in The Daily Telegraph.
He began by declaring: "It would be a travesty if the democratic verdict of the 2016 referendum – the largest in British history – were not delivered, yet the prime minister's proposed 'deal' is so bad that it cannot be considered anything other than a betrayal of clear manifesto promises."
And he concluded by dismissing her whistlestop tour to win concessions, claiming: "No amount of tinkering will yield a majority in Parliament for this deal."
He added: "Throughout this process, I have sought to support the government. The conclusion is now inescapable that the Prime Minister is the blockage to the wide-ranging free trade agreement offered by Tusk which would be in the best interests of the country and command the support of parliament."
The leader of the Brexiteer European Research Group of MPs, Jacob Rees-Mogg, stopped short of claiming Sir Graham has received 48 letters, after being embarrassed last month when his exaggerated claims proved inaccurate and he was accused of leading a "Dad's Army plot".
But defending his campaign to remove Mrs May and his letter to Sir Graham demanding a vote, Mr Rees-Mogg told Sky News: "It doesn't require a general election.
"But historically prime ministers have gone when they can't get their major business through the House of Commons and therefore I think it's Mrs May's duty to go to the Palace and offer the Queen her resignation."
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Meanwhile, the bookies' favourite to succeed Mrs May as leader, former foreign secretary Boris Johnson, whose new neater haircut led to speculation that he was poised to stand, reveals in a Spectator column that he has lost 12lb in two weeks "after receiving a wake-up call" about his health.
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