Theresa May tells Tories she won't lead them into 2022 General Election

However, it is a secret ballot and the level of the rebellion could be crucial to her survival.

One loyal Tory MP said that anything above 100 would make life very difficult for Mrs May while a rebel suggested 80 was the key figure as this represented half of the backbench MP tally.

The televised result in Committee Room 14 in the Commons is due at 9pm.

Failure in the ballot would trigger a leadership contest in which Mrs May could not stand. Yet if she won, another challenge could not be mounted against her position as Tory leader for a year.

However, Labour made clear that the PM’s comments about not leading the Conservatives into the 2022 General Election meant, even if she survived tonight’s vote, she would be a lame duck.

In a packed and emotionally-charged meeting of the Tories’ 1922 backbench committee just before voting started, Mrs May told colleagues that she wanted to fight the next election in three and a half years’ time but now accepted they did not want her to.

MPs attending the meeting said that at times the PM became emotional. At various points tables were banged in support.

Most contributions were said to have been supportive of Mrs May, the most negative came from one backbencher, who declared: “Stamina is not a policy.”

As MPs flooded out of the meeting, the vast majority of comments were positive.

Amber Rudd, the Work and Pensions Secretary, said: "She's made the commitment that is what people wanted but she was very clear that she won't be taking the General Election in 2022."

Robert Buckland, the Solicitor-General, told reporters: "She said: 'In my heart I would like to lead the party into the next election' and then that was the introductory phrase to her indication that she would accept the fact that that would not happen, that is not her intention."

One backbencher said there was a huge amount of support for and goodwill towards the party leader.

“She said her focus was to deliver on Brexit and create a strong and stable government. The PM also stressed she had to come back with something from the EU to get the DUP on board.”

A Scottish Tory MP, loyal to the PM, told The Herald: “It went as well as could be expected. When she talked about the election and her 40 years in the party she said get a bit emotional.”

Another Scot said: “It was a very strong performance, a very emotional performance. She certainly did enough to win over the room. I expect her to win tonight, we just have to make sure it’s by a big enough margin. She made clear she is not going to be leading us into the next election.”

Brandon Lewis, the party Chairman, said the PM had been “very frank” given MPs’ views about the 2022 election.

“I’m confident she can do it. It’s the right thing to do to make sure we keep Theresa May as PM, so we can get through these next few months, get Brexit done and get back to delivery for people on the domestic agenda,” he explained.

But Jacob Rees-Mogg, the ardent Brexiteer who leads the anti-EU European Research Group[ERG], was less convinced. “After the election, she came in and took the room by storm. It was a triumph for her and have people confidence. This was a very half-hearted room in comparison with June 2017.”

He made clear the PM had not convinced him to vote for her. "It was all the same old stuff. Nothing has changed."

Asked if he thought she would lose the vote, the Somerset MP replied: “Predicting a result hours before it happens, I did that at the last election and I was completely wrong. I’m not going to make the same mistake again.”

Labour MPs also seized upon Mrs May's comments about the 2022 election, suggesting it was clear that she would not remain long in Downing Street.

"Even if she wins, this duck is lame," said former minister David Hanson.

Mrs May was informed that she would face a ballot by the chairman of the 1922 Committee, Sir Graham Brady, in a phone call at 10.35pm on Tuesday.

She had just returned to Downing Street from a day of travels which had taken her to The Hague, Berlin and Brussels for Brexit talks with EU leaders.

As day broke in Westminster on Wednesday, Sir Graham issued a press release announcing that the threshold had been reached and a confidence vote would be held later that day.

In a dramatic early morning statement outside the door to No 10, Mrs May declared: "I will contest that vote with everything I've got."

Warning that a change of prime minister would put the UK's future at risk and could delay or halt Brexit, she insisted she would stay on to "finish the job".

Mrs May said securing a Brexit deal which would deliver on the result of the 2016 referendum was "now within our grasp" and said she was "making progress" in securing reassurances from EU leaders on MPs' concerns about the proposed backstop for the Irish border.

Husband Philip showed his support by watching Prime Minister's Questions from the public gallery, while Tory elder statesman Kenneth Clarke told MPs that a leadership contest would be "irresponsible and unhelpful".

None of the PM's Brexit-backing critics took the opportunity to attack her, while there were strong words of support from backbenchers including Neil O'Brien, who condemned "headbangers from all sides" for undermining her.

But she faced a call to stand down from the Scottish National Party's leader in Westminster, Ian Blackford, who said: "This Government is a farce, the Tory Party is in chaos and the Prime Minister is a disgrace... Prime Minister, take responsibility, do the right thing, resign."

Mrs May cleared the decks to lobby Tory MPs for their backing, cancelling a planned meeting of Cabinet and a trip to Dublin for talks with Irish premier Leo Varadkar.