BRITISH Prime Minister Theresa May has won a confidence vote against her after vowing to fight efforts to oust her from the job.

The party room erupted in cheers when Conservative party chairman Graham Brady declared the party "does have confidence in Theresa May as leader". Ms May won the vote 200-117.

Earlier Ms May warned outside 10 Downing St that a change of Prime Minister would put the UK's future at risk and could halt Brexit.

She declared she would stay on to "finish the job," but later told Tory MPs that she would not be leading the Conservative party into the next general election.

Three black metal boxes were taken to a room for counting after Conservative politicians spent two hours voting earlier today.

The more than 300 paper ballots they cast were counted by hand.

Ms May needed a majority vote to hold onto her position as prime minister and Conservative Party leader.

Earlier, at least 175 Conservative Party politicians indicated support for Ms May as they headed into the vote, based on public statements in the media and social media.

Ms May needed to secure a simple majority - 158 of 315 politicians - to remain leader.

In a dramatic development Ms May said she would stand down before the next election, after she has steered Brexit through.

"In my heart I would like to lead the party into the next election, but I accept that won't happen," she said.

Some ministers were reportedly in tears after the PM's announcement.

Ms May refused to put an exact date on when she will stand down despite calls for her to go as soon as Britain formally leaves the EU.

In a defiant press conference outside 10 Downing Street, Ms May said changing Conservative leaders would "put our country's future at risk and create uncertainty when we can least afford it" and could lead to Brexit being delayed or prevented.

"Weeks spent tearing ourselves apart will only create more division," the PM said, warning that leadership challenge threatens to hand power to Labour.

"I stand ready to finish the job."

The PM said any new leader would have to extend the March 29 deadline for Britain's exit from the European Union.

"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," Ms 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."