Prime Minister Theresa May has vowed to fight for her job in a leadership challenge triggered by Conservative lawmakers, saying a change could jeopardise Britain's divorce from the European Union.

Key points: The vote will be held between 5:00am and 7:00am AEDT on Thursday

The vote will be held between 5:00am and 7:00am AEDT on Thursday Mrs May said she will contest the leadership ballot

Mrs May said she will contest the leadership ballot Mrs May warned MPs that Brexit would need to be delayed if she loses the vote

A vote of confidence on Mrs May's leadership will be held between 6:00pm and 8:00pm local time on Wednesday (5:00am-7:00am AEDT) in the House of Commons and an announcement made as soon as possible afterwards, according to Graham Brady, chairman of the party's so-called 1922 committee.

Mr Brady said he received 48 letters from Conservative MPs indicating their loss of confidence in Mrs May, the required amount to trigger a vote.

Speaking outside her Downing Street residence hours before the vote, Mrs May said she would battle for her premiership with everything she had.

"Through good times and bad over the last two years, my passionate belief that a [Brexit] deal is attainable remains intact."

"A change of leadership in the Conservative Party now would put our future at risk.

"The British people want us to get on with it. We must and we shall deliver on the referendum vote.

"I have devoted myself unsparingly to these tasks since I became Prime Minister, and I stand committed to the job."

The odds may be in her favour

Theresa May delayed a parliamentary vote on her Brexit deal, conceding she would have lost. ( Reuters: Yves Herman )

According to the rules, Mrs May could be toppled if a simple majority — 158 Conservative lawmakers — vote against her, though a significant rebellion could also undermine her position.

At least 153 of her 315 Conservative lawmakers had expressed public support for her by midnight AEDT.

However, some lawmakers who have backed Mrs May publicly have said in private that they will vote against her, according to British political commentators.

If Mrs May survives the no confidence vote, she would be safe from any further leadership challenges for a full 12 months.

If she loses, Mrs May is not able to run in the leadership ballot.

Brexit will be delayed if May loses

In a stark warning to Brexit-supporting opponents in her party, Mrs May said that if they toppled her then Brexit would have to be delayed and perhaps even stopped.

"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," Mrs 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," she said.

She said a change of leadership "would put our country's future at risk and create uncertainty when we can least afford it".

"Weeks spent tearing ourselves apart will only create more division just as we should be standing together to serve our country," Mrs May said.

Justice Minister David Gauke had earlier told the BBC that Britain would need to delay its exit from the EU if Mrs May lost the vote, saying "it is inevitable that if she were to lose the vote tonight there would be need to be a delay in Article 50".

With less than four months until the UK is due to exit the EU, on March 29, the world's fifth-largest economy is tipping towards crisis, opening up the prospect of a disorderly no-deal divorce or a reversal of Brexit through a second referendum.

The ultimate outcome will shape Britain's economy, have far-reaching consequences for the unity of the United Kingdom and determine whether London can keep its place as one of the top two global financial centres.

The British pound, which has lost 25 cents against the US dollar since the 2016 referendum, fell after news of the confidence vote broke but then rose to $US1.2548 after revelations Brexit might have to be delayed.

Conservative Party split on May

Brexit-supporters in her party have accused Mrs May of selling out Britain in negotiations.

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"Theresa May's plan would bring down the Government if carried forward," Conservative MPs Jacob Rees-Mogg and Steve Baker said in a statement.

"But our party will rightly not tolerate it. Conservatives must now answer whether they wish to draw ever closer to an election under Mrs May's leadership. In the national interest, she must go."

But at least 100 Conservative Party MPs expressed support for Mrs May ahead of the confidence vote, based on public statements in the media and social media.

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"I am backing @theresa_may tonight," Foreign Secretary Jeremy Hunt said on Twitter.

"Being PM most difficult job imaginable right now and the last thing the country needs is a damaging and long leadership contest."

Home Secretary Sajid Javid also tweeted Mrs May had his "full support".

"The last thing our country needs right now is a Conservative Party leadership election. Will be seen as self-indulgent and wrong," Mr Javid tweeted.

Those who could put themselves in the running for the leadership include former foreign secretary Boris Johnson, ex-Brexit secretary David Davis, or International Trade Secretary Penny Mordaunt, among others.

Mr Davis said he would listen to a speech by Mrs May to a meeting of conservatives later on Wednesday before declaring whether he will support her in the vote of no confidence.

He said that he would not support a counterproductive move to delay article 50, suggesting that the EU likes to negotiate at the last minute.

Following the news of the vote, Mrs May cancelled a meeting of her Cabinet of senior ministers which had been scheduled for later on Wednesday, her office said in a statement.

The Labour Party slammed Mrs May, with party chairman Ian Lavery saying her weakness "has completely immobilised the Government at this critical time for the country".

"The Prime Minister's half-baked Brexit deal does not have the backing of her Cabinet, her party, Parliament or the country," Mr Lavery said in a statement.

"The Conservative Party's internal divisions are putting people's jobs and living standards at risk."

ABC/wires