UK Prime Minister Boris Johnson and US President Joe Biden are expected to agree on an "Atlantic Charter" laying out their plans when they meet each other for the first time on Thursday.
The charter is named after the 1941 pact between wartime leaders Winston Churchill and Franklin D Roosevelt that led to the creation of Nato.
It will seek to reinvigorate the transatlantic alliance by pledging co-operation on climate change and recovery from Covid-19.
Meeting on the eve of the G7 summit in Britain, the two leaders will also seek an agreement on resuming UK-US travel after the pandemic.
But their encounter is taking place in the shadow of warnings from Washington that Britain must avoid jeopardising the Northern Ireland peace process.
Britain and the EU, who accuse each other of failing to honour an agreement on cross-border checks, failed to make a breakthrough during post-Brexit discussions on Wednesday.
European Commission President Ursula von der Leyen said on Thursday that the protocol was the "one and only solution" and that she saw "fundamental gaps" in Britain's implementation of it.
"We are determined to do everything to keep peace and stability on the island of Ireland. It is important that there is deep respect for the protocol," she said.
America's charge d'affaires to the UK, Yael Lempert, reportedly told Brexit Minister David Frost that the UK's stance on the Northern Ireland Protocol was inflaming tension in Ireland.
Ms Lempert is said to have issued Mr Frost with a demarche, a formal diplomatic reprimand, at a meeting in London on June 3.
A demarche is more commonly lodged with adversaries than close allies.
According to a leaked government memo obtained by The Times, Ms Lempert said the dispute over the implementation of post-Brexit arrangements in Northern Ireland was "commanding the attention" of Mr Biden, who is a proud Irish-American.
She relayed to him Mr Biden’s “great concern” over the UK’s approach to the protocol, which was established to prevent a hard Irish border.
It said that the US had urged the UK to come to a “negotiated settlement” with the EU, even if that meant making “unpopular compromises”.
Mr Biden's national security adviser Jake Sullivan said on Wednesday that any steps to undermine peace in Northern Ireland "would not be welcomed by the United States".
Asked whether Mr Johnson was jeopardising peace, Mr Sullivan said: "I'm not going to characterise that at this point. I'm only going to say that President Biden is going to make statements in principle on that front."
In a move likely to provoke anger from Brexiteers, the US reportedly suggested that if the UK signed up to EU rules on agricultural standards to ease problems with the protocol, Mr Biden would ensure it did not obstruct a UK-US trade deal.
The UK has ruled out aligning with the EU’s food and animal health rules, saying this would bind it to laws set in Brussels.
On Wednesday night, UK sources confirmed the details of the meeting but insisted Britain and the US were united in their determination to maintain peace in Northern Ireland.
Asked whether the G7 summit would be overshadowed by the spat, Mr Johnson said: "I'm not worried about that."
The prime minister said his charter with Mr Biden would cover science, technology, trade and defence.
Above all, it will underscore "our joint commitment to Nato that has been indispensable to our security for decades", he said.
"The time has come to dispel any sense of gloom and show how Nato is looking ahead to 2030," he said.
The charter is expected to call for urgent action on climate change, biodiversity and recovery from the Covid pandemic.
The two leaders are expected to agree in principle on a technology agreement that would allow both countries to share expertise in areas such as artificial intelligence.
On transatlantic travel, they are expected to set up a task force to make recommendations on resuming flights.
"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," Mr Johnson said.
"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."
After talks with Mr Johnson, Mr Biden will attend the G7 gathering in a Cornish seaside resort from Friday to Sunday.
He will then visit Queen Elizabeth II at Windsor Castle and fly to Brussels for summits with Nato and the EU next week.
He will finish in Geneva to meet Russia's President Vladimir Putin next Wednesday.