Boris Johnson has claimed he can force the EU to deliver a fresh Brexit deal because it is terrified by the “existential threat” of Nigel Farage, as he finally agreed to a major interview.

The clear favourite for No 10 said the solution was “obvious” – to strip the Irish backstop out of the divorce agreement – and that Brussels would fold, despite its repeated refusal to do so.

“I think what they will see is that politics has changed in the UK and in Europe, they have now 29 Brexit MEPs in Strasbourg,” Mr Johnson argued – while again vowing to deliver a no-deal Brexit on 31 October, if the EU refused.

Speaking to the BBC, he denied he had taken cocaine since he was a teenager, while admitting to a “single inconclusive event” as a youth.

And he claimed anyone accusing him of blundering over the jailing of Nazanin Zaghari-Ratcliffe in Iran was excusing “the people who are really responsible – and that is the Iranian Revolutionary Guard.”

Mr Johnson bowed to pressure to face detailed questioning after growing criticism that he was hiding from scrutiny as the runaway favourite coasting to near-certain victory in the race.

Who is standing to be the next prime minister? Show all 3 1 /3 Who is standing to be the next prime minister? Who is standing to be the next prime minister? Boris Johnson - 157 votes Foreign Secretary Boris Johnson arriving in Downing Street, London, for a Cabinet meeting where Theresa May will brief on her plans for Brexit before a major speech aimed at helping to break the deadlock in the negotiations with Brussels. PA Who is standing to be the next prime minister? Michael Gove - 61 votes LONDON, ENGLAND - JUNE 07: Britain's Environment Secretary Michael Gove leaves 10 Downing Street on June 7, 2018 in London, England. Prime Minister Theresa May is holding an emergency Brexit cabinet meeting in an attempt to resolve tensions over the UK's Irish border plan. (Photo by Simon Dawson/Getty Images) Getty Who is standing to be the next prime minister? Jeremy Hunt - 59 votes Health and Social Care Secretary Jeremy Hunt leaving Downing Street, London, after attending a Cabinet meeting. PA

He said he would take part in a BBC debate among the surviving candidates on Tuesday, but not the Channel 4 event on Sunday evening – before further voting knocks out some rivals.

Doubling down on his threat to carry out a crash-out Brexit on Halloween night if necessary, Mr Johnson insisted: “I don’t think some of the promises of doom and disaster are true.”

However, he claimed it would be necessary, because the EU would shift on the backstop faced with the “existential threat” of the Brexit Party’s dramatic rise.

It would agree to replace the backstop – the guarantee of no return to a hard border – by agreeing checks could take place elsewhere, using new technology.

“There is a clear way that the now effectively defunct withdrawal agreement can be disaggregated,” he claimed, “the good bits of it can be taken out.”

Ian Murray, a Labour supporter of the People’s Vote campaign immediately ridiculed the suggestion, saying: “If the definition of insanity is trying the same thing over and over again and expecting different results, then this proposal is the craziest of all.

“The EU has already made it crystal clear that the current extension was granted on condition that there would be no effort to change the withdrawal agreement.

“It is not going to tear up it up for prime minister Boris Johnson, or any other prime minister for that matter.”

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Mr Johnson refused to backtrack on 31 October as the deadline for leaving, deal or no deal, saying it was time “to get this thing done”.