British Foreign Secretary Boris Johnson has said concerns over a hard border between Ireland and the UK are "pure millennium bug stuff", in comments captured in a recording obtained by BuzzFeed News.

"All the planes crashing from the sky. It's absolute nonsense," he said in unguarded comments at a private dinner.

Mr Johnson, who was speaking at the gathering of the Conservative Way Forward, a Thatcherite campaign group, said exit talks were approaching a "moment of truth".

He said fears about the border were out of proportion.

"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," he said.

"We're allowing the whole of our agenda to be dictated by this folly".

The cabinet minister was speaking to around 20 people dining in a private room after a reception at the Institute of Directors on Wednesday night.

Mr Johnson warned of a Brexit "meltdown" and branded the UK Treasury the "heart of Remain". He said he believed Brexit would happen and would be "irreversible" but the "risk is that it will not be the one we want".

He said British Prime Minister 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".

Mr Johnson suggested Chancellor Philip Hammond's department was "basically the heart of Remain".

He also revealed he was "increasingly admiring of Donald Trump".

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"I have become more and more convinced that there is method in his madness," he added.

"Imagine Trump doing Brexit. 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".

A friend of Mr 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".

Labour's Ian Murray, a supporter of the People's Vote campaign for a second referendum, said: "Boris Johnson is utterly shameless.

"As these comments show, he knows we're going to get shafted with a bad Brexit deal, he knows how much damage it will cause to our economy, he knows it will result in a hard border re-emerging in Northern Ireland, but he couldn't care less.

"That this disgraceful charlatan holds one of the great offices of state in this country should be a source of constant shame and embarrassment to the prime minister. Boris Johnson cares about nothing except himself and his own ambition.

"He is right about one thing: we are heading for 'a meltdown' over Brexit. But his solution is simply to embrace the chaos".