Conservative leadership rivals Boris Johnson and Jeremy Hunt are due to debate head-to-head for the first time in front of a live TV audience.
Party members are voting on which of the two men should succeed Theresa May as leader and prime minister, with the winner due to be announced on 23 July.
The men have been taking part in hustings across the UK but this is the first time they have shared a stage.
The hour-long debate is being broadcast on ITV from 20:00 BST.
They will face questions from members of the public, including Tory activists, on Brexit and other issues.
The 180,000 or so Conservative members eligible to take part have already begun voting by post, after ballot papers started arriving at people’s homes on Friday.
It will be the first time a sitting prime minister has been chosen by party members.
Mr Hunt had been pressing for the TV debate to take place earlier, warning that many people will have made up their minds before seeing the two men cross-questioned on live TV.
The show, entitled Britain’s Next Prime Minister: The ITV Debate, will be hosted by journalist Julie Etchingham in front of studio audience of 200 people at MediaCityUK.
After short opening statements, the two men will face questions from ITV viewers submitted in advance and selected by the broadcaster.
Mr Johnson, the former Mayor of London, is regarded as the clear frontrunner in the race, having won the support of more than 50% of Tory MPs in the first phase of the contest.
But Mr Hunt, the current foreign secretary, has in recent days won the heavyweight endorsements of former leaders Sir John Major and William Hague.
Both men have said they would prefer to leave the EU on 31 October with a negotiated deal but are willing to get out without a legal agreement if necessary.
During two weeks of campaigning, both have made bold spending promises and offers of tax cuts – which have been criticised by financial experts and Chancellor Philip Hammond.