Boris Johnson has said he is "not aiming for a no-deal outcome" for Brexit at the launch of his campaign for the Tory leadership.
But he said leaving no deal on the table was a "vital tool of negotiation" and the UK "must do better than the current withdrawal agreement".
"Delay means defeat, delay means Corbyn," he said, saying the UK must leave the EU on October 31.
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 June 25, 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 for the job of Conservative party leader - and prime minister - 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.
At Wednesday's launch, the former foreign secretary - who quit over Mrs May's Brexit policy - said it was "right for our great country to prepare" for a no-deal outcome.
He said any delay to Brexit will "further alienate not just our natural supporters but anyone who believes that politicians should deliver on their promises".
And he warned his party it would "kick the bucket" if it went into the next election having failed to carry the mandate given to it by the British people.
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 Organization terms if necessary while it negotiates a future trade deal.
Earlier, 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."
On June 18, BBC One will host a live election debate between the Conservative MPs still in the race.
If you would like to ask the candidates a question live on air, use the form below. It should be open to all of them, not a specific politician.