US national security adviser Jake Sullivan confirmed the president did hold a “candid discussion” with the prime minister about the issue at the G7 summit in Cornwall.
Boris Johnson had claimed after his Thursday meeting with Mr Biden that they were in “complete harmony” on Northern Ireland – and No 10 insisted Mr Biden had not raised concerns about protocol arrangements.
But Mr Sullivan said the president had indeed expressed his views on the issue with “deep sincerity” – suggesting Mr Biden may have delivered his message with some feeling.
“All I’m going to say: they did discuss this issue. They had a candid discussion of it in private,” Mr Sullivan said late on Sunday, answering questions from American reporters.
The White House adviser added: “The president naturally, and with deep sincerity, encouraged the prime minister to protect the Good Friday Agreement and the progress made under it. The specifics beyond that, I’m not going to get into.”
Mr Sullivan would not be drawn on whether the president had linked the issue to a free trade deal with the UK, and did not specify when the conversation took place.
The two leaders held a bilateral meeting on Thursday before the main summit began and afterwards Mr Johnson played down any differences between them on the issue.
The disclosure came as the gathering ended with a furious diplomatic spat after foreign secretary Dominic Raab accused French president Emmanuel Macron of talking about Northern Ireland “as if it was some kind of different country to the UK”.
At his closing news conference, Mr Macron strongly denied that he had ever questioned British sovereignty and insisted the UK must honour its commitments under the Brexit deal.
Mr Johnson is said to have teased Mr Macron about French military prowess on Sunday. The prime minister said the wine he gave to Biden “was as old as when the French last won a naval battle,” according to The Sun.
The continuing row over the protocol – intended to protect the peace process by ensuring there is no return to a hard border with the Republic – overshadowed much of the summit.
Mr Johnson repeated his warning that he could unilaterally delay the legally agreed checks on chilled meats moving from Great Britain to Northern Ireland at the end of the month unless new arrangements are agreed.
The EU has previous said that its patience is wearing “very, very thin” and had threatened to launch a trade war unless the UK abides by its treaty obligations.
The Republic of Ireland’s foreign minister Simon Coveney claimed on Sunday there was still a “middle ground” when it came to aligning food standards, saying a compromise could reduce some checks on food imports from GB to NI.
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