British Prime Minister Theresa May is waiting to find out her fate following a confidence vote fate following shambolic Brexit negotiations.

The secret ballot has now closed and the announcement is due at 9pm local time (8am AEST).

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

Voting is now underway in the British Parliament on Theresa May's future as leader. (AAP)

Earlier, in a fiery speech outside 10 Downing Street, Mrs May said she was determined to "finish the job" of negotiating the UK's divorce from the European Union and warned removing her as leader risks the future of the country. (AAP)

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

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

If she is defeated, she will have to resign, although she will remain as a caretaker prime minister until a new Conservative leader is chosen.

May had a smile on her face as she arrived back at 10 Downing Street after the vote.

However, Bookmakers have suspended betting on the ballot as momentum moved in favour of the prime minister.

Mrs May has appeared relaxed and even in good spirits today as she faced a fiery question time in parliament hours before the vote of confidence in her leadership.

Despite the chaotic day, Mrs May, who has vowed to fight the challenge "with everything I've got", appeared jovial as she faced a barrage of attacks during Prime Minister's Questions in parliament, smiling as she quipped her day would be full of "many meetings".

Brexit protesters in London. (AAP)

(AAP)

Earlier, in a fiery speech outside 10 Downing Street, Mrs May said she was determined to "finish the job" of negotiating the UK's divorce from the European Union and warned removing her as leader risks the future of the country.

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

"A new leader wouldn't have time to renegotiate a (Brexit) withdrawal agreement and get the legislation through Parliament.

File photos of possible contenders for British prime minister if Theresa May loses a vote of confidence (clockwise from top left) Michael Gove, Jeremy Hunt, Amber Rudd, David Davis, Sajid Javid, Dominic Raab, Jacob Rees-Mogg, and Boris Johnson. (AAP) (AAP)

"One of their first acts would have to be extending or rescinding Article 50 - delaying or even stopping Brexit when people want use to get on with it.

"Weeks spent tearing us 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 agenda I set out in my first speech outside this front door - delivering the Brexit people voted for, building a country that works for everyone. I have devoted myself unsparingly to these tasks ever since I became Prime Minister and I stand ready to finish the job."

The no-vote has been triggered by the letters of at least 48 Conservative lawmakers, reaching the threshold of 15 percent of the party seeking a vote.

The country's politics has been thrown into chaos and Brexit into doubt after Conservative members triggered a no-confidence vote in Mrs May that will see her removed as party and government leader if she loses. (AAP)

If she loses the confidence vote, Mrs May must step down and there will be a contest to choose a new leader. She will remain leader, and prime minister, until the successor is picked. If she wins, she can't be challenged again for a year.

May has reportedly told Tory MPs she will not lead the party to the next election if she wins a vote of no confidence against her.

An hour before MPs were due to start casting their votes, Mrs May held a meeting where the BBC reports she told her party she had wanted to lead them to the next election but the ballot today was, for now, not about the future.

Several leading Brexiteers, including former Foreign Secretary Boris Johnson and ex-Brexit Secretary Dominic Raab, have said loudly that they think they could get a better deal with the EU, and are likely to enter a race to replace her.

More conciliatory candidates including Home Secretary Sajid Javid and Work and Pensions Secretary Amber Rudd, are also possible contenders.

If all Tory lawmakers cast ballots, Mrs May needs 158 votes to win.

(AAP)

Voting is now underway on British Prime Minister Theresa May’s fate following shambolic Brexit negotiations. (AAP)

There are reports she already has enough support to win, assuming those who have said publicly they will vote for her follow through on the anonymous ballot.

A narrow victory could leave her weakened and under pressure to resign, but allies say Mrs May should stay in post even if she secured a wafer-thin win.

"I think she needs to win by one," said International Trade Secretary Liam Fox.

Former Environment Secretary Owen Paterson accused Mrs May of acting like a "supplicant" in dealings with the EU.

"She's not the person to see Brexit through," he said.

Leading pro-Brexit legislators Jacob Rees-Mogg and Steve Baker said in a joint statement that "in the national interest, she must go."

But Cabinet colleagues rallied to Mrs May's support. Home Secretary Sajid Javid tweeted that a leadership contest, with Brexit little more than three months away, "will be seen as self-indulgent and wrong. PM has my full support and is best person to ensure we leave EU on 29 March."

Justice Secretary David Gauke said: "I think it's vital for the country that she wins tonight."

He said that if Mrs May lost, "I don't think we will be leaving the European Union on the 29th of March."

With Associated Press