Joe Biden has flown into the UK ahead of a meeting with Boris Johnson on Thursday on his first overseas trip as US president.
Mr Biden will warn Boris Johnson and the European Union not to “imperil” the Northern Ireland peace process, the US president’s national security adviser said, as they struggle to resolve a trade row.
The president will also use the visit to strengthen ties with allies at the G7 summit on Friday and join efforts to agree a strategy to vaccinate the world against coronavirus.
Mr Biden flew in to the UK on Air Force One, landing at the US airbase at RAF Mildenhall before heading on to Cornwall for Thursday’s talks with Mr Johnson and the G7 summit in Carbis Bay.
Ahead of the talks with Mr Johnson, Mr Biden’s national security adviser Jake Sullivan warned that the president harbours “very deep” concerns on the issue provoked by Brexit.
Mr Sullivan said the president believes the post-Brexit Northern Ireland Protocol is “critical” to ensuring that the Good Friday Agreement is protected, as Britain and the EU try to resolve the issue of checks in the Irish Sea.
The adviser told the BBC that both sides must continue with negotiations, adding: “But whatever way they find to proceed must, at its core, fundamentally protect the gains of the Good Friday Agreement and not imperil that.
“And that is the message that President Biden will send when he is in Cornwall.”
Mr Sullivan declined to say whether a failure to alleviate the Northern Ireland concerns could jeopardise a transatlantic trade deal, but added that “our concern runs very deep on the Northern Ireland issue”.
But the Prime Minister said he is “not worried” about the prospect of the summit being overshadowed by the row with the EU.
Asked about Mr Sullivan’s warning, Mr Johnson told broadcasters in Cornwall that he was “very, very optimistic” about the situation and said a solution is “easily doable”.
Mr Biden and his wife Jill will kick off the eight-day trip to Europe by meeting US personnel stationed at the Mildenhall airbase in Suffolk.
Having succeeded Donald Trump after his single fractious term in the White House, Mr Biden said the trip – which also takes in a Nato meeting and talks with Russia’s Vladimir Putin – was aimed at smoothing relations with allies which had been strained under his predecessor.
He told reporters the aim was “strengthening the alliance, making it clear to Putin and China that Europe and the United States are tight”.
Mr Biden has promised the meeting with the Prime Minister would “affirm the special relationship between our nations”.
However, this week it emerged Mr Johnson is not a fan of the expression, after an American magazine reported he told aides he does not like the seemingly “needy and weak” term.
Downing Street said he “prefers not to use the phrase” and is instead intent on fostering a “close relationship” with Washington.
Part of that, Mr Johnson hopes, is a trade deal with the US. However, the national security adviser’s remarks were the latest in a string of warnings about Brexit’s impact on Northern Ireland.
The Prime Minister plans to use the summit of leading economies to urge the members – also including Canada, Japan, France, Germany and Italy – to “defeat” Covid-19 by helping to vaccinate the world by the end of next year.
The US President told reporters that he would be announcing a vaccine strategy for the world.
Mr Johnson also wants a new treaty on responding to pandemics after the “pretty scratchy period” where countries were competing and “squabbling” over access to personal protective equipment.
The summit would focus on “building back greener, building back better” after the pandemic, he said.
After the summit ends, the president and first lady will meet the Queen at Windsor Castle.
Mr Biden will then depart for Brussels and later Geneva in Switzerland.