Boris Johnson has described working with US President Joe Biden as "a big breath of fresh air" following the pair's first face-to-face meeting - during which the two leaders appeared to skirt around a confrontation over Northern Ireland.
The prime minister met Donald Trump's successor in Carbis Bay on Thursday, ahead of this week's G7 summit at the Cornwall resort.
Mr Johnson said the new US administration had "so much they want to do together", including on NATO and climate change - subjects on which Mr Biden has dramatically different viewpoints to Mr Trump.
And the prime minister also suggested a potentially strained conversation with the US president over post-Brexit arrangements in Northern Ireland had been avoided, as he hailed the "common ground" between the UK, US and EU on preserving the Good Friday Agreement.
It follows reports Mr Biden's administration had accused Mr Johnson's government of "inflaming" tensions during an ongoing row between the UK and EU over the implementation of the Northern Ireland Protocol.
Downing Street did not deny the UK had received a "demarche" - a formal diplomatic protest - from the US over the dispute, but said Mr Johnson and Mr Biden were in "complete harmony" on the issue.
After his discussions with the US president, which lasted around an hour and 20 minutes, Mr Johnson said: "The talks were great, they went on for a long time, we covered a huge range of subjects.
"It's wonderful to listen to the Biden administration and to Joe Biden, because there's so much they want to do together with us, from security, NATO, to climate change.
"And it's fantastic, it's a breath of fresh air."
Asked whether the US president - who has often spoken about his Irish roots - had made his alarm about the situation in Northern Ireland clear, Mr Johnson replied: "No he didn't.
"But what I can say is America, the US, Washington, the UK - plus the EU - have one thing we absolutely all want to do; that is to uphold the Belfast Good Friday Agreement and make sure we keep the balance of the peace process going.
"That's absolutely common ground and I'm optimistic we can do that."
Shadow foreign secretary Lisa Nandy called for the Brexit issue to be "resolved", adding: "It is hard to think of a prime minister who is going into the G7 summit as the host in a worse state."
"I think the prime minister just needs to resolve these things. It shouldn't take the President of the United States to tell the prime minister that he ought to uphold the peace agreement in Northern Ireland - that ought to be an article of faith for any UK prime minister," she told Sky News.
Mr Johnson and Mr Biden had been due to meet at Saint Michael's Mount, an historic castle on a tidal island off the coast of Cornwall.
Reports suggested their wives, Carrie Johnson and Jill Biden, were scheduled to have a tour of the island while the leaders held talks.
But a Number 10 source earlier confirmed the visit to Saint Michael's Mount was "sadly off due to the weather" and the talks were rearranged to Carbis Bay, where the G7 summit will be held between Friday and Sunday.
Prior to their meeting, Mr Johnson, Mr Biden and their wives stood together to enjoy the view across the bay.
"It's gorgeous, I don't want to go home," Mr Biden said.
And, after Dr Biden and Mrs Johnson left the two leaders, the US president revealed how he told the prime minister "we have something in common, we both married way above our station".
Newlywed Mr Johnson replied that he was "not going to dissent from that point, I'm not going to disagree with the president there".
"Or indeed on anything else, I think it highly likely," he added.
Dr Biden - who enjoyed time on the beach on Thursday with Mrs Johnson and her baby Wilfred - wore a jacket embroidered with the word "LOVE" on the back, an item the first lady previously wore at the kickoff rally of her husband's presidential campaign in 2019.
Asked about her outfit, Dr Biden said: "I think that we're bringing love from America.
"This is a global conference and we're trying to bring unity across the globe and I think that's important right now - that people feel a sense of unity... feel a sense of hope after this year of the pandemic."
Some immediately drew comparisons between Dr Biden's jacket and her predecessor as first lady, Melania Trump, who wore a green jacket that said, "I Really Don't Care, Do U?" on a 2018 trip to a Texas border town to visit migrant children housed in shelters.
Before getting to the substance of their talks, Mr Biden and Mr Johnson had inspected documents related to the Atlantic Charter, a declaration signed by Winston Churchill and Franklin D Roosevelt in August 1941 that set out common goals for the world after the Second World War.
And, following their discussions, it was announced the US and UK had agreed a "New Atlantic Charter" to build upon those commitments and aspirations set out 80 years ago.
In his own comments after the talks with Mr Johnson, Mr Biden confirmed America's commitment to purchasing 500 million doses of the Pfizer COVID vaccine to donate to poorer countries.
He said the meeting with Mr Johnson had been "very productive" and said the revitalised Atlantic Charter would address the "key challenges of this century - cyber security, emerging technologies, global health and climate change".
"We affirmed the special relationship - that is not said lightly - the special relationship between our people and renewed our commitment to defending the enduring democratic values that both our nations share," the US president said.
Mr Johnson was recently reported to dislike the phrase "special relationship" and to believe it makes the UK seem "needy and weak".
To mark their first meeting, Mr Johnson presented Mr Biden with a framed photograph of a mural of Frederick Douglass, a former slave who became a leading figure in the 19th century abolitionist movement in the United States.
In return, the prime minister - a keen cyclist - was given a US-made bike and helmet.
Dr Biden was given a first edition of Daphne du Maurier's The Apple Tree, which Downing Street said was to reflect the author's Cornish links.
And Mrs Johnson received a leather tote bag made by military wives and a presidential silk scarf.