* May's opponents trigger confidence vote

* Ballot at 1800-2000, result due 2100 GMT

* PM to fight party leadership challenge

* May says a new leader could delay Brexit

* More than 120 Conservatives indicate support (Adds Merkel and Fox)

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

Less than four months before the United Kingdom is due to leave on March 29, Brexit is in chaos with options ranging from a potentially disorderly no-deal departure to another referendum that could reverse it.

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

In a stark warning to Brexit-supporting opponents who instigated the challenge, May said if they toppled her then the EU exit would be delayed and perhaps even stopped.

A new leader would not have time to renegotiate a deal with the EU and secure parliamentary approval by the end of March, meaning the Article 50 withdrawal notice would have to be extended or rescinded, she said.

"A change of leadership in the Conservative Party now would put our country's future at risk and create uncertainty when we can least afford it," she said

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

A secret ballot will be held between 6pm and 8pm local time on Wednesday (5am and 7am AEDT Thursday) in a room at the House of Commons, and an announcement made at 9pm (8am AEDT).

Before the vote, May will speak to MPs at a closed meeting.

According to the rules, May could be toppled if a simple majority of Conservative MPs vote against her, though a significant rebellion could also undermine her position. At least 153 of her 315 Conservative MPs had expressed public support for her by 1pm.

Brexit-supporting MPs in her party say May has betrayed the people's vote in negotiations, while opponents say she struck a deal that is the worst of all worlds - out of the EU but with no say over many rules it has to abide by.

"Theresa May's plan would bring down the government if carried forward," 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."