Boris Johnson has warned any further delay to Brexit will spell disaster for the Conservatives ahead of the launch of his leadership campaign.
He will tell 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 will say 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.
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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.
Who will replace Theresa May?
The winner of the contest to lead the Conservative Party will become the next prime minister.
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.”
‘Delivering on promises’
At Wednesday’s launch, the former foreign secretary – who quit over Mrs May’s Brexit policy – will say 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.
For three simple reasons Boris Johnson’s campaign team has been very deliberately trying to closet him away in comparison to the others.
Because, well, if you’re ahead why do anything that might mess it up? This is still the early stages of a campaign, which is going to go on for some weeks, and they expect he will get to the final stages.
Their time has been more valuably spent trying privately to get MPs on board.
And even his biggest supporters know Boris Johnson has the capacity to implode.
So while he is going to be out there today, I think he’s not going to be suddenly spilling everything and we’re going to hear a very straightforward political message.
The case he will make is he’s the only candidate in this contest who can hold off Nigel Farage on the one side and have the cut-through and appeal to keep Labour at bay on the other, in the way he did as mayor of London.
Others, including Michael Gove and Mark Harper, have indicated they would be prepared to seek a further extension from the EU to finalise a better deal.
Brexiteers such as Dominic Raab and Esther McVey have said the priority must be honouring the 2016 referendum result and the UK should be prepared to trade with the EU on World Trade Organization terms if necessary while it negotiates a future trade deal.
Mr Johnson will suggest the huge task of uniting the country can only begin once the UK has left the EU and government has delivered on the “one big thing” asked of it by the public.
And he will warn his party it will “kick the bucket” if it goes into the next election having failed to carry the mandate given to it by the British people.
“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. Delay means defeat. Delay means Corbyn.”
On Tuesday 18 June BBC One will host a live election debate between the Conservative MPs still in the race.
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Tory leadership: Boris Johnson says there can be no further Brexit delay}