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The US President praised the closeness of ties between the two countries as he and the Prime Minister held talks in the garden of No 10.
Mr Biden, whose short layover in London comes ahead of a crunch Nato summit in Vilnius, Lithuania, said the “relationship is rock solid”.
He said he “couldn’t be meeting with a closer friend and a greater ally”, while Mr Sunak hailed the US and UK as “two of the firmest allies” in Nato.
But the issue of support for Ukraine’s ambitions for Nato membership and the US decision to provide Kyiv with cluster munitions are signs that Westminster and Washington are not entirely on the same page.
The president in recent days defended the “difficult” decision to send the weapons to Ukraine, with Mr Sunak responding by saying Britain “discourages” their use as one of 123 signatories of a convention banning the bombs
No 10 confirmed that the issue was discussed in the meeting between the leaders, which lasted around 40 minutes and was also attended by US secretary of state Antony Blinken and Foreign Secretary James Cleverly.
The Prime Minister’s official spokesman said providing the weapons “was a difficult choice for the US” that had been “forced on them by Russia’s war of aggression”.
The two leaders “discussed the commitments that UK has under that convention, both not to produce or use cluster munitions and to discourage their use”.
The UK is signed up to an international convention to ban the weapons, placing Mr Sunak under a duty to speak out against their use, but the US is not signed up to that agreement.
The spokesman said that Mr Sunak complied with the UK’s international commitments and that the two men “discussed the requirements the Prime Minister is under because of this convention, and the UK is upholding that”.
The Nato summit is also likely to see wrangling between allies over Ukraine’s path to membership of the alliance.
Though all attendees at the summit in Vilnius on Tuesday agree that Ukraine cannot join during the war, a move which would pull the wider West into direct conflict with Russia, the US is seen as most hesitant over its membership.
Downing Street played down reports that the UK and US were split over the extent of commitments to offer Ukraine on Nato membership.
Mr Biden has described Kyiv’s bid as “premature”, telling CNN: “I don’t think it’s ready for membership in Nato.”
Britain, on the other hand, has indicated support for a fast-track approach for Ukraine.
The Prime Minister’s spokesman said it would be a topic for discussion at the Nato gathering.
“Certainly, we do want to support Ukraine on the pathway to joining the alliance, the exact mechanisms for that are for discussion with Nato allies,” the spokesman said.
The spokesman rejected suggestions there was a difference between the US and UK positions on the issue.
Mr Biden, who appeared relaxed in the Downing Street garden, ignored shouted questions from reporters and kept the conversation focused on the sunny relationship between the two countries.
The latest meeting comes weeks after Mr Sunak travelled to Washington, where the US and UK announced the “Atlantic Declaration” to bolster economic security.
Mr Biden also made a brief trip to Northern Ireland earlier this year to mark the 25th anniversary of the Good Friday Agreement.
The White House has not been shy in the past in expressing concern about the impact of Brexit on the peace process, with the leaders briefly discussing the importance of the 1998 peace accord during Monday’s talks.
Though it is not a full-blown state visit, Mr Biden has also been treated to a display of pageantry at Windsor Castle, where he received a warm welcome from the King.