Second vote in race to succeed Theresa May
Conservative MPs will vote by secret ballot in the Commons, with a result expected some time after 18:00 BST.
Any of the six remaining candidates will be eliminated from the contest if they come last or fail to secure at least 33 votes.
Those remaining in the race will take part in a live BBC debate in central London on Tuesday evening.
Remaining candidates will face further ballots later this week, where the bottom-ranked MP will be knocked out until only two candidates are left.
The final two names will then be put to a postal vote of the 160,000 Tory party members, beginning on 22 June, with the winner expected to be announced about four weeks later.
Former Foreign Secretary Boris Johnson remains the clear frontrunner in the race after topping the first ballot earlier this month with 114 votes.
Former cabinet minister Andrea Leadsom – who was knocked out of the race at the first ballot – has given Mr Johnson her support.
Speaking on LBC, she said Mr Johnson was “an election winner” who was “best placed” to take the UK out of the EU by 31 October.
Current Foreign Secretary Jeremy Hunt, who came second with 43 votes, and Environment Secretary Michael Gove, who was third with 37, should make it through to the next ballot on Wednesday if their support holds firm.
Former Brexit Secretary Dominic Raab, who received 27 votes, and Home Secretary Sajid Javid, who received 23, told reporters on Monday they were confident of making it through to the next round.
International Development Secretary Rory Stewart, who received 19 votes in the first ballot, said he had the necessary 33 backers to stay in the race “if they do what they say”.
One candidate will definitely be knocked out of the competition, quite possibly more, because they have got to get 33 votes.
That probably puts Sajid Javid and Dominic Raab on difficult terrain.
As for Rory Stewart, the signs are that he is now beginning to build up some real momentum – not just outside Parliament but among MPs too.
He has set himself out as the soft Brexit, no no-deal candidate but also as the anti-Boris Johnson candidate.
Other contenders have started to pull their punches, looking at life beyond this contest to a Boris Johnson government. Rory Stewart is still taking punches at him.
For all those Tory MPs who want to stop Boris Johnson, he becomes a very attractive candidate.
As for Mr Johnson, tonight’s debate will be his first serious date with detail.
We will want detail on whether he is absolutely 100% going to leave the EU on 31 October or whether there is a bit of wriggle room.
Mr Stewart, who is currently sixth-placed among the remaining contenders, received a boost to his campaign on Monday evening with the endorsement of Cabinet Office Minister David Lidington.
Mr Lidington, who had backed Matt Hancock before he quit the race last week, told a rally for Mr Stewart there was a “yearning in this country for political leaders who tell it straight to people”.
Speaking on BBC Breakfast, Mr Stewart claimed the majority of Mr Hancock’s supporters were now backing him.
He added that Mr Lidington’s backing was important as it was a “vote of confidence in somebody he feels could run a cabinet and be prime minister”.