At the gathering of the Conservative Way Forward, a Thatcherite campaign group, he said exit talks were approaching a “moment of truth”. The foreign secretary said he believed Brexit will happen and would be “irreversible” but the “risk is that it will not be the one we want”.

In comments captured in a recording obtained by BuzzFeed News, he said Theresa May was “going to go into a phase where we are much more combative with Brussels”. He added: “You’ve got to face the fact there may now be a meltdown. OK? I don’t want anybody to panic during the meltdown. No panic. Pro bono publico, no bloody panic. It’s going to be all right in the end.”

Johnson suggested the chancellor Philip Hammond’s department was “basically the heart of remain”.

The foreign secretary also said that he was “increasingly admiring of Donald Trump” and has become “convinced that there is method in his madness”, according to a leaked recording from a private dinner for Tory activists.

“Imagine Trump doing Brexit,” Johnson said, according to the recording leaked to BuzzFeed News. “He’d go in bloody hard … There’d be all sorts of breakdowns, all sorts of chaos. Everyone would think he’d gone mad. But actually you might get somewhere. It’s a very, very good thought.”

After he had delivered the keynote speech at the group’s summer reception, at the Institute of Directors in London, a select group – including Conor Burns, the MP for Bournemouth West and the foreign secretary’s parliamentary private secretary – were said to have retired to a private room for dinner.

He revealed that the US wants Britain to “use our nuclear expertise to dismantle Kim Jong-un’s nuclear missile”.

“That’s what [Mike Pompeo, the US secretary of state] asked me to do today,” Johnson said – a revelation that is likely to raise questions about the discretion of Britain’s top diplomat.

A friend of Johnson said: “This was a private dinner under Chatham House rules so it is sad and very disappointing that it has been covertly recorded and distributed to the media.”

On Brexit, Johnson told activists that the European commission is playing hardball during negotiations in order to prove it is not worth trying to leave to the other 27 member states.

He also questioned the weight laid on the contentious Irish border issue in the government’s approach to the negotiations.

“It’s so small and there are so few firms that actually use that border regularly, it’s just beyond belief that we’re allowing the tail to wag the dog in this way. We’re allowing the whole of our agenda to be dictated by this folly,” he said.

On China, Johnson said the UK needs to engage with the emerging superpower while recognising they remain commercial rivals.

“We need to engage with China diplomatically, treat China as our friend and our partner, but also recognise that they are our commercial rivals. And they will try to stiff us.”

Johnson predicted that Chinese companies would eventually take over from Silicon Valley as the world’s technology giants.

“The Americans have run the tech world for decades. Microsoft, Google, Apple, blah, blah, blah – we’re used to them winning,” the foreign secretary said.

“No, no, no. The Chinese are about to win. They’ve got 5G. They’ve found out a way. Everybody’s going to be getting stuff on their gizmos through the Chinese system and not the American system. So watch out for that one.”

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