Boris Johnson allegedly joked "f*** the Americans" over calls to back Rishi Sunak's Brexit agreement to ease relations with the White House.
Claims of the ex-Prime Minister's four-letter tirade came as his successor Mr Sunak battled to get a new agreement on the Northern Ireland Protocol over the line.
Mr Johnson, who used his final No10 speech to urge his successor to "stay close the Americans", reportedly made the remark in a private discussion in the Commons chamber.
According to The Sunday Times, the former PM was urged by Sir Robert Buckland, a minister in his Cabinet, to back Mr Sunak's new deal.
Last week Mr Johnson refused to say whether he would support the proposals and insisted his law-breaking Northern Ireland Protocol Bill was better.
It was reported Sir Robert told his former boss that a deal on the post-Brexit trading arrangements for Northern Ireland was necessary to boost US relations.
"Boris said 'f*** the Americans!", the report added.
A source close to the ex-PM told The Mirror: "This was a jocular conversation in the chamber with Robert Buckland that someone has evidently misunderstood. This is not language we would use".
Sir Robert described the exchange as "cordial" to the Mail on Sunday.
But Labour's Shadow Northern Ireland Secretary Peter Kyle said: "It's no joke to the people of Northern Ireland who are desperate to get a new deal done".
It came as the Deputy Prime Minister Dominic Raab said Mr Sunak is on "the cusp" of securing new Brexit terms with the EU.
He said an agreement was expected in "days, not weeks".
Mr Raab, asked on Sky News' Sophy Ridge On Sunday programme whether a deal could be unveiled as soon as Monday, replied: "I think there is real progress.
"We want to make sure all the pieces are in place.
"I think, hopefully, there will be good news in a matter of days, not weeks."
The Cabinet minister later told the BBC's Laura Kuenssberg: "If we can get this over the line - we're on the cusp, we've made great progress, we're not there yet - this would be a really important deal."
Speaking to The Sunday Times, Mr Sunak pledged that "anything that we do will tick all of those boxes" in terms of Unionist concerns with the protocol.
The protocol, signed by Mr Johnson in 2020, was designed to prevent a hard border with Ireland after Brexit by effectively keeping Northern Ireland in the EU's single market.
But the trade barriers created by the treaty has created Unionist tension, with Mr Sunak admitting that it had "unbalanced" the Good Friday Agreement that helped end the Troubles bloodshed in Northern Ireland.
