Boris Johnson has warned any further delay to Brexit will spell disaster for the Conservatives at the launch of his leadership campaign.
He told supporters the UK must leave on 31 October and any more “can-kicking” will mean defeat for the party at the next general election.
He said his two election victories as London mayor mark him out as the person most able to beat Jeremy Corbyn.
Home Secretary Sajid Javid will also make his pitch to MPs on Wednesday.
In a campaign video released on Tuesday, Mr Javid said he would bring a “fresh look” to the Conservative Party and was capable of healing the divisions caused by Brexit.
Mr Javid has won the backing of Scottish Conservative leader Ruth Davidson, who will introduce him at Wednesday’s event.
Meanwhile Labour has tabled a cross-party motion to stop a no-deal Brexit being pushed through by a future prime minister.
It plans to force a vote on Wednesday to give MPs control of the timetable on 25 June, so MPs would be able to introduce legislation to avoid a no-deal scenario.
Mr Johnson and Mr Javid are the last of the 10 candidates in the contest to officially launch their campaigns ahead of Thursday’s first ballot of Tory MPs.
Mr Johnson is regarded as the frontrunner in the contest, with many more endorsements from Tory MPs than any other candidate.
He has kept a low profile in the race to succeed Theresa May so far, his only significant intervention being a pledge – immediately criticised by his rivals – to cut income tax bills for people earning more than £50,000 a year.
Chief Secretary to the Treasury Liz Truss defended Boris Johnson from accusations he is refusing to appear in media interviews during his leadership bid.
“He has got nothing to hide,” she told Radio 4’s Today programme.
“The important thing is he is talking to parliamentary colleagues.”
At Wednesday’s launch, the former foreign secretary – who quit over Mrs May’s Brexit policy – said the UK “must” leave the EU on 31 October.
“We simply will not get a result if we give the slightest hint that we want to go on kicking the can down the road with yet more delay,” he will say.
“With every week and month that goes by in which we fail to deliver on our promise I am afraid we will further alienate not just our natural supporters but anyone who believes that politicians should deliver on their promises.”
Several of Mr Johnson’s rivals, including Rory Stewart and Matt Hancock, have said they would not countenance leaving the EU at all without some form of legally-binding agreement because of the economic disruption it would cause.