Donald Trump was called on Air Force One last year by a prankster posing as Piers Morgan, the TV presenter says.
The president, as he was at the time, only realised he had been tricked when he phoned the real Morgan while on his way to vote in Florida last year.
The alleged security breach is said to have happened in October, but only emerged in an interview Morgan gave to the BBC's Americast podcast.
The two recently had a falling out over Mr Trump's handling of the pandemic.
Asked by the BBC's Jon Sopel why Mr Trump had called Morgan out of the blue this past October, the presenter described "an absolutely hilarious story, where somebody had called [Trump] pretending to be me the day before and got through to him on Air Force One".
The 45th US president didn't realise he had been duped, Morgan said. "They had a conversation with Trump thinking he was talking to me."
It is not clear who the alleged hoaxers were, but if the story is true President Trump would not be the first political leader to have been pranked.
Canadian Prime Minister Justin Trudeau and British Prime Minister Boris Johnson, while he was foreign secretary, have both been tricked on the phone in recent years.
But it would revive long-running questions about the security of President Trump's phone conversations.
The BBC has asked the Secret Service for comment.
Morgan was a high-profile tabloid editor in the UK who took over from Larry King with a primetime CNN chat show in 2011. He now presents a breakfast show in the UK.
He was initially supportive of President Trump after his surprise election win but became increasingly critical in the last 12 months.
"We had a very nice conversation... I always got on well with Trump," Morgan said of their October call, but added that Mr Trump's "character flaws - the chronic narcissism, the desire to make everything about himself" made him a "useless leader".
On their friendship, Morgan described Mr Trump's behaviour since the November presidential election as "egregious" and "so obviously on a pathway" to the Capitol Hill riots on 6 January.
"I just felt - no, I'm done with you now," Morgan said.
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