Joe Biden will tell Boris Johnson not to let the row over Northern Ireland’s Brexit arrangements put the Good Friday Agreement at risk when the pair meet on Thursday.
In the US president’s first overseas visit, aides said he will stress the need to “stand behind” the Northern Ireland Protocol, the element of the Brexit deal which has triggered a UK-EU dispute.
The issue has threatened to overshadow the Prime Minister’s first meeting with the president and his hosting of the G7 summit, which begins on Friday in Cornwall.
Aside from Brexit, Mr Johnson and Mr Biden will work on efforts to resume transatlantic travel and agree a new Atlantic Charter paving the way for co-operation on challenges including climate change and security.
But Mr Biden’s close interest in issues affecting Ireland will mean that the dispute over the protocol will feature heavily in discussions with the UK and European Union over the coming days of intense diplomatic activity in Cornwall.
The Times reported that the president – who is intensely proud of his Irish roots – took the extraordinary step of ordering the United States’ most senior diplomat in London, Yael Lempert, to deliver a demarche – a formal protest – in a meeting with Brexit minister Lord Frost on June 3.
The newspaper reported that Government minutes of the meeting said: “Lempert implied that the UK had been inflaming the rhetoric, by asking if he would keep it ‘cool’.”
The US charge d’affaires indicated that if Mr Johnson accepted demands to follow EU rules on agricultural standards, Mr Biden would ensure that it would not “negatively affect the chances of reaching a US/UK free trade deal”.
Talks between Brexit minister Lord Frost and the European Commission’s Maros Sefcovic on Wednesday failed to make a breakthrough on the protocol.
The EU has threatened to launch a trade war against Britain if it fails to implement checks on goods entering Northern Ireland under the terms of the Brexit “divorce” settlement which Mr Johnson signed.
Lord Frost refused to rule out the prospect that the UK could unilaterally delay imposing checks on British-made sausages and other chilled meats due to come into force at the end of the month.
At a press conference in Brussels on Thursday ahead of the summit, European Commission president Ursula von der Leyen again insisted the protocol was the “only solution” to prevent a hard border with the Republic and must be implemented in full.
“We have shown flexibility, we will show flexibility, but the protocol and the (Brexit) Withdrawal Agreement have to be implemented completely,” she said.
The protocol effectively keeps Northern Ireland in the European single market for goods in order to avoid a hard border with Ireland, meaning a trade barrier in the Irish Sea for goods crossing from Great Britain.
Mr Biden’s national security adviser Jake Sullivan told reporters on Air Force One: “President Biden has been crystal clear about his rock-solid belief in the Good Friday Agreement as the foundation for peaceful coexistence in Northern Ireland.
“That agreement must be protected, and any steps that imperil or undermine it will not be welcomed by the United States.”
Asked whether Mr Johnson’s stance was imperilling the peace deal, Mr Sullivan said: “I’m not going to characterize that at this point. I’m only going to say that President Biden is going to make statements in principle on this front.
“He’s not issuing threats or ultimatums; he’s going to simply convey his deep-seated belief that we need to stand behind and protect this protocol.”
Mr Johnson told reporters on Wednesday that resolving the dispute with Brussels was “easily doable” and “what we want to do is make sure that we can have a solution that guarantees the peace process, protects the peace process, but also guarantees the economic and territorial integrity of the whole United Kingdom”.
The meeting of the two leaders comes on the eve of the G7 summit which will bring together the world’s wealthiest democracies at a time when the West faces difficult judgments in responding to the rise of China as an economic and political force and the destabilising actions of Russia.
As part of that process, the new Atlantic Charter will commit the UK and US to apply their combined strength to the enormous challenges facing the planet today, including global defence and security, “building back better” from coronavirus, and stopping climate change.
The original Atlantic Charter was a joint statement made by Winston Churchill and Franklin D Roosevelt in 1941, setting out the UK and US goals for the post-Second World War world.
Mr Johnson said: “While Churchill and Roosevelt faced the question of how to help the world recover following a devastating war, today we have to reckon with a very different but no less intimidating challenge – how to build back better from the coronavirus pandemic.
“And as we do so, co-operation between the UK and US, the closest of partners and the greatest of allies, will be crucial for the future of the world’s stability and prosperity.”
No immediate breakthrough is expected on resuming travel between the US and UK, which ground to a halt as the pandemic hit but a new joint taskforce will work on the issue.
Mr Biden used a speech to military personnel shortly after Air Force One arrived in the UK at RAF Mildenhall to outline the change in approach to international affairs following the Donald Trump era.
He told an audience of US military personnel and service families: “We are going to make it clear that the United States is back and democracies of the world are standing together to tackle the toughest challenges and the issues that matter most to our future.”
It comes as Pfizer and BioNTech said 500 million doses of their vaccine have been sent to the US government, which is donating them to around 100 low- and lower-middle income countries.
The companies are offering the doses at a not-for-profit price.