The National Security Adviser to the US President has said the president has concerns over the Northern Ireland Protocol.
Ahead of talks between Mr Biden and British Prime Minister Boris Johnson in Cornwall tomorrow, Jake Sullivan said the president believes the Protocol is "critical" to ensuring that the Good Friday Agreement is protected.
Speaking on Air Force One ahead of President Biden's arrival in the UK, Mr Sullivan said the US had been "crystal clear" on the need for the Good Friday Agreement.
He told reporters: "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."
After being asked if Mr Johnson was "taking steps to imperil" the agreement, he added: "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 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."
Asked if @BorisJohnson is taking steps to imperil the GFA, @JakeSullivan46: "I'm not going to characterize that at this point… @JoeBiden is not issuing threats or ultimatums; he's going to simply convey his deep-seated belief that we need to stand behind & protect this protocol"— Brian O'Donovan (@BrianOD_News) June 9, 2021
The British 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".
The comments come following the latest round of talks between the UK and the EU over the issue of checks on goods crossing the Irish Sea.
The European Union warned Britain that it would react swiftly if it acted alone to change post-Brexit trading arrangements for Northern Ireland, but did not spell out what its steps might be.
The United States 'is back'
President Biden stressed the need for the world's democracies to stand together as he arrived in the UK on his first overseas trip as US president.
The president will seek to show the United States is back as a major international diplomatic force following the Donald Trump years, using the visit to Europe to shore up transatlantic alliances.
Mr Biden flew in to the UK on Air Force One, landing at the US airbase at RAF Mildenhall.
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."
He called for international action to tackle climate change and end the Covid-19 pandemic.
"This diplomacy is essential because no single nation acting alone can meet all the challenges we face today because the world is changing," Mr Biden said.
The US President also quoted Irish poet WB Yeats upon arrival, from the poem Easter 1916: "All changed, changed utterly: A terrible beauty is born."
Addressing US troops at the RAF Mildenhall in Suffolk, President Joe Biden quoted WB Yeats poem 'Easter 1916' - 'All changed, changed utterly; A terrible beauty is born' | https://t.co/bUAdHeKOQj pic.twitter.com/3JbbUfunmq— RTÉ News (@rtenews) June 9, 2021
Mr Biden's trip to Europe also takes in a NATO meeting and talks with Russia's Vladimir Putin "to let him know what I want him to know".
The G7 meeting is expected to be dominated by vaccine diplomacy, trade, climate and an initiative for rebuilding infrastructure in the developing world.
US officials see that effort as a way to counter China's growing influence.
Mr Biden may face pressure to do more to share US vaccine supplies with other countries after an initial pledge of 20 million doses announced last week.
His push for a global minimum tax on multinational corporations faces opposition at home.
G7 finance ministers agreed before the summit to pursue a global minimum tax rate of at least 15% and to allow market countries to tax up to 20% of the excess profits - above a 10% margin - generated by about 100 large, high-profit companies.
US President Joe Biden departs for Britain today on his first trip abroad since taking office.— RTÉ News (@rtenews) June 9, 2021
Next week he will become the fifth US President to meet Russian President Vladimir Putin | Read more: https://t.co/xnuefqwaMO pic.twitter.com/X0bUHLXXLD
Republicans came out against the plan this week, potentially complicating the US ability to implement a broader global agreement.
After three days of G7 summitry, Mr Biden and his wife Jill will visit Queen Elizabeth at Windsor Castle.
Mr Biden, 78, met the queen back in 1982 when he was a US senator from Delaware.
Afterwards, Mr Biden travels to Brussels for talks with leaders of NATO and the European Union.
The agenda is expected to be dominated by Russia, China and the perennial issue of getting NATO allies to contribute more to the common defence.
Mr Biden closes out the trip in Geneva for what could prove to be the most difficult meeting of the week - a session with Mr Putin, who had enjoyed friendly relations with Mr Trump.
White House national security adviser Jake Sullivan told reporters that Mr Biden hoped his G7 and NATO meetings will bolster a sense of allied unity as he goes into his session with Mr Putin.
No major breakthroughs are expected from the summit. Mr Sullivan said Mr Biden would press Mr Putin on US priorities.
The two sides were negotiating whether to hold a joint news conference.