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The five remaining candidates to become Tory leader and prime minister are facing a third round of voting by Conservative MPs.
The lowest-placed contender will be eliminated, with the result expected to be announced at 20:00 BST.
Ex-Chancellor Rishi Sunak has come top in the two previous rounds, followed by Trade Minister Penny Mordaunt and Foreign Secretary Liz Truss.
A TV debate between the contenders on Sky News on Tuesday has been cancelled.
But the rivals have been speaking at a hustings organised by Tory MPs ahead of the vote.
Separately, a vote of confidence in the government will take place in the House of Commons on Monday evening, with ministers expected to see off any challenge by opposition MPs.
The Tory leadership contest has become increasingly bitter and personal in tone, as those left in it fight for the final two places, which will allow them to go forward to a summer-long contest, in which Conservative Party members vote for their favoured candidate.
Backbench MP Tom Tugendhat, who came fifth in the last round of MPs' votes, has rejected calls to stand aside, as has former Equalities Minister Kemi Badenoch, who came fourth.
But most of the rancour has been between Mr Sunak, Ms Mordaunt and Ms Truss, who clashed in an ITV-hosted debate on Sunday over their economic policies.
Tuesday's Sky debate was cancelled after Mr Sunak and Ms Truss declined to take part, and amid concern among senior Tories that angry arguments in public could damage the party.
'Blue-on-blue'
Speaking at Monday's hustings, Mr Tugendhat warned against allowing Labour to benefit from Conservative "disunity and division".
"I have deplored the infighting, the sniping, the blue-on-blue, the attacks and the smears," he said. "We do not need this, now or ever, because we are fighting on every flank."
At the same event, Ms Truss stressed her "heavyweight" credentials, according to a source from her campaign.
A supporter of Mr Sunak said they did not expect to pick up many extra votes in Monday's ballot, with many of those who had backed the last eliminated candidate - Attorney-General Suella Braverman - instead opting for the ex-chancellor's rivals.
Ms Badenoch said she was "always" confident about her prospects as she left the hustings.
And Ms Mordaunt said the event had been "all very good, all very jolly".
The vote, involving up to 358 Tory MPs, lasts until 19:00.
The Commons vote of confidence in the government - prompted by pressure from Labour - could lead to a general election if the government is defeated.
But this is highly unlikely, as it would require dozens of Conservative MPs to side with Labour and other opposition parties.
The result of this is expected at about 22:00.