"Johnson has never wanted to accept that Biden is an Irish-American Catholic who cares deeply about the island of Ireland," Peter Westmacott, the former UK Ambassador to the US, has written.

Westmacott was responding after a report emerged that Boris Johnson said "f--- the Americans" when being urged to support moves by Prime Minister Rishi Sunak and the EU to solve Brexit problems over the Northern Ireland Protocol, which are supported by the US President.

Johnson has led the opposition to Sunak's Northern Ireland deal, which significantly reduces customs checks for goods entering Northern Ireland from Great Britain. 

The Biden administration has made it clear that there will be no post-Brexit trade deal between the US and the UK until political disputes in Northern Ireland have been resolved.

According to Westmacott, Biden was "long suspicious of the whole Brexit process" because, in part, of his concerns about how it could impact the Good Friday Agreement.

"Like many others who follow the Irish question, the president and his staff quickly realized that the Northern Ireland protocol, negotiated and then disowned by the then prime minister [Johnson], was not a credible solution," Westmacott wrote.

Last week, when it appeared that Sunak and the EU were nearing a new deal on the Northern Ireland Protocol, further reports emerged that Johnson was keen to derail the efforts.

"Tossing aside American goodwill and support at this latest critical stage of efforts to 'make Brexit work' would be very unwise," Westmacott wrote.

"Clearly the current prime minister [Sunak] has no intention of behaving so irresponsibly, not to mention jeopardizing a presidential visit.

"It would help if his predecessor-but-one stopped making a difficult job impossible, and offending our closest ally and partner in the process, for reasons that appear to have little to do with the interests of the people of Northern Ireland."

According to the UK Independent, Johnson made the "f--- the Americans" remark during a heated exchange with former Lord Chancellor Sir Robert Buckland, who urged Johnson to support Sunak's efforts to restore the Protocol and help smooth relations with the US.

"It’s vital you back it Boris, it’s a good outcome, and with the US state visit coming up and the need to get a trade deal with them it will help keep us onside with the Americans," Buckland told Johnson, according to a witness to the exchange. 

"F*ck the Americans," Johnson reportedly responded. 

Ultimately, Sunak and EU Commission President Ursula von der Leyen unveiled their plan, dubbed the Windsor Framework, on Monday, February 27. President Biden and other American politicians welcomed the development.

Johnson has criticized Sunak's Northern Ireland deal and has suggested that he will not vote for it. 

The former British Prime Minister said the deal with the EU was not about the UK "taking back control."

"This is the EU graciously unbending to allow us to do what we want to do in our own country, not by our laws, but by theirs."

Up to 100 Conservative MPs could refuse to back the deal, which would leave Sunak in the embarrassing position of requiring help from the Labour Party. 

Further, if the issue remains unresolved, Biden could nix his rumored visit to the island of Ireland in April.