Boris Johnson has admitted he would need EU co-operation to avoid a hard Irish border or crippling tariffs on trade in the event of no deal.
In an exclusive interview with the BBC, the favourite to be next prime minister said: “It’s not just up to us.”
But he said he did “not believe for a moment” the UK would leave without a deal, although he was willing to do so.
Asked about a row he had with his partner, he said it was “simply unfair” to involve “loved ones” in the debate.
Reports of the argument on Friday with his girlfriend, Carrie Symonds, dominated headlines over the weekend after the police were called to their address in London.
The interview comes after Sky News said it would have to cancel a head-to-head debate on Tuesday between the two leadership contenders as Mr Johnson had “so far declined” to take part.
Work and Pensions Secretary Amber Rudd told Radio 4’s Today programme she found Mr Johnson’s decision to ignore live TV debates “very odd” and urged him “to reconsider”.
Meanwhile, the other candidate, Jeremy Hunt has promised to boost defence spending by £15bn over the next five years if he becomes prime minister.
Johnson said the existing deal negotiated by Theresa May “is dead”.
He insisted it was possible to broker a new deal with the EU before the end of October because the political landscape had changed in the UK and on the continent.
“I think actually that politics has changed so much since 29 March,” he said, referring to the original Brexit deadline.
“I think on both sides of the Channel there’s a really different understanding of what is needed.”
At the moment, the UK is due to leave the EU on 31 October after the PM’s Brexit deal was rejected three times by Parliament.
Mr Johnson said he would be able to persuade Brussels to resolve the Irish border issue – a key sticking point – despite repeated warnings from EU leaders that that was impossible.
He said there were “abundant, abundant technical fixes” that could be made to avoid border checks.
When challenged that these do not exist yet, Mr Johnson replied: “Well, they do actually, you have in very large measure they do, you have trusted trader schemes, all sorts of schemes that you could put into place.”
But, he admitted, there was “no single magic bullet” to solve the issue.