Theresa May has vowed to fight for her job, warning Brexit would have to be delayed or even cancelled if she has toppled by Conservative MPs.

Speaking in Downing Street, the prime minister announced she would contest the no-confidence vote of all Tory MPs tonight “with everything I’ve got”.

And, in a dramatic warning to her MPs not to desert her, she raised the prospect of a new leader being forced to delay or annul Brexit, because they would not have time to negotiate a new deal.

“One of their first acts would have to be extending, or even rescinding, Article 50 - delaying, or even stopping, Brexit when people want us to get on with it,” Ms May said.

The expected pledge to fight the contest came one hour after Sir Graham Brady, the chairman of the 1922 backbench committee, announced the vote of no confidence would go ahead.

Sir Graham released a statement saying the threshold of 48 signatures – 15 per cent of the total number of Tory MPs – “has been exceeded”.

The contest will be staged between 6pm and 8pm this evening, when all 315 Conservative MPs will vote on whether they believe Ms May should continue.

The prime minister requires 158 supporters to survive – after which there cannot be another contest for one year – although rejection by more than 100 Tory MPs could damage her fatally.

A new Tory leader – and prime minister – would not be in place until the middle of January at the very earliest, depending on the number of hustings to be staged for party members.

Furthermore, grassroots Tories are almost certain to pick a Brexiteer, who could put the UK on course for a no-deal Brexit if the EU refuses to renegotiate.

Outside No 10, Ms May urged her MPs to recognise that forcing her out would “put our country’s future at risk and create uncertainty when we can least afford it”.

She argued: “A new leader wouldn’t be in place by 21 January legal deadline, so a leadership election risks handing control of the Brexit negotiations to opposition MPs in parliament.

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“The new leader wouldn’t have time to renegotiate the withdrawal agreement and get the legislation through parliament by 29 March, 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.”

Ms May also warned that “a leadership election would not change the fundamentals of the negotiation, or the parliamentary arithmetic”.

“Weeks spent tearing ourselves apart will only create more division just as we should be standing together to serve our country.

“None of that would be in the national interest. The only people whose interests would be served would be Jeremy Corbyn and John McDonnell.”

Ms May also announced she was cancelling a trip to Dublin - and a cabinet meeting - this afternoon, to argue for a better Brexit deal, to deal with the crisis.

Ian Lavery, the Labour party chairman, claimed the Conservatives’ “internal divisions are putting people’s jobs and living standards at risk”.

"With only weeks left before Britain leaves the EU, Theresa May's weakness and failure has completely immobilised the government at this critical time for the country,” he said.

Arlene Foster, the leader of the Democratic Unionist Party (DUP) – which is propping up the Tories in power – said she was not “surprised” by the dramatic events.