Conservative MPs will begin the process of choosing their next leader and UK prime minister, when the first round of voting gets under way later.
Theresa May goes in to the ballot with a lead among Tory MPs.
But energy minister Andrea Leadsom, who won Boris Johnson’s backing on Monday, Michael Gove, Stephen Crabb and Liam Fox also hope to succeed David Cameron.
Party members will choose from the two backed by most Tory MPs, with the winner due to be named on 9 September.
The Conservative Party’s 330 MPs will vote in the first round of the election between 11:00 BST and 18:00 BST, with the results expected at about 19:00 BST, when the poorest performing candidate will be eliminated.
Further rounds will take place on Thursday and the following Tuesday until two candidates remain.
On Monday the candidates were given the chance to make their cases to be the next Conservative leader during a parliamentary hustings.
During the meeting, Mrs May is understood to have repeated her view that the status of EU nationals currently in Britain would form part of negotiations ahead of Britain’s withdrawal from the EU.
Mrs May’s rivals, Mr Gove and Mrs Leadsom, have said EU citizens should have their right to stay in the UK protected.
After the meeting, one Tory MP opposed to Mrs Leadsom’s bid to become leader described an attempt to distance herself from UKIP support as a “car crash”.
But Boris Johnson insisted she had drive and described her as “kind and trustworthy” as he backed her campaign.
The contest has been sparked by Mr Cameron’s decision to step down as prime minster after the UK voted by 52% to 48% to leave the EU, in the June referendum.
The PM, who had campaigned strongly for a Remain victory, said “fresh leadership” was required as the UK negotiated its exit from the European Union.
Mr Johnson, the former mayor of London and a leading figure in the Brexit campaign, had been expected to run for the Tory leadership.
But his leadership hopes were dashed after Mr Gove launched his own bid for the job, with a stinging attack on his fellow Leave campaigner.
By Laura Kuenssberg, BBC political editor
As MPs prepare to vote in the first leadership ballot, Tory minds are starting to focus on what life, post-David Cameron will look like.
The expectation is that the two names on the ballot that goes to Tory party members for the final decision will be Theresa May and Andrea Leadsom.
But despite their victory 10 days ago that will change the country, the Outers are struggling to find a stand-out candidate.
Michael Gove’s standing after last week’s Machiavellian moves has suffered. Andrea Leadsom has a long way to go to convince MPs and members that she is ready for the job.
Right now it’s someone who was on the losing side in the referendum, Theresa May, that is well in front.
But given how quickly things have changed in Westminster in just a few short days, making any predictions is a risky game.
On Monday, Mrs May, the home secretary who campaigned to stay in the EU, was out in the lead among her colleagues, with the declared support of 115 MPs – about 35% of the parliamentary Conservative Party.
Among her backers are former supporters of Mr Johnson, including cabinet minister Liz Truss and former shadow home secretary David Davis.
Mrs Leadsom has 40 declared supporters – including Mr Johnson and Northern Ireland Secretary Theresa Villiers.
Mrs May and Mrs Leadsom are followed by Mr Gove, the justice secretary, with 26, Mr Crabb, the work and pensions secretary, on 23 and Mr Fox, a former defence secretary, on nine.
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