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 that 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 coveted backing of Scottish Conservative leader Ruth Davidson, who will introduce him at Wednesday’s event.
The two men 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.
Who will replace Theresa May?
The winner of the contest to lead the Conservative Party will become the next prime minister.
Foreign Office minister Andrew Murrison and veteran Eurosceptic Sir Bill Cash are the latest to pledge their support on Tuesday.
Mr Johnson 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.
‘Delivering on promises’
At Wednesday’s launch, the former foreign secretary – who quit over Theresa May’s Brexit policy last summer – will suggest that he would take the UK out of the EU by the revised deadline of 31 October, come what may.
“After three years and two missed deadlines, we must leave the EU on October 31.
“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.
“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.
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 Organisation terms of 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 that 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.”
Mr Johnson ran for the leadership in 2016 but pulled out before the first round of voting after Mr Gove, who was at the time his campaign manager, withdrew his support, questioning his aptitude and appetite for the job.