US President Joe Biden will fly into the UK on Sunday amid tensions over his decision to send cluster bombs to Ukraine.
Air Force One is expected to touch down at Stansted Airport this evening before Mr Biden meets with both Rishi Sunak in Downing Street and the King at Windsor Castle on Monday.
Mr Biden’s stopover in Britain comes ahead of a crunch two-day NATO summit in Vilnius, Lithuania, on Tuesday, in which leaders are set to discuss the war in Ukraine and the future of the military alliance as several nations clamber to join.
But it comes as controversy grows over the White House’s announcement that it will send cluster munitions to Kyiv, despite use of the bombs being banned by 123 nations across the world – including Britain.
Mr Biden, who will be accompanied on his European tour by his Secretary of State Antony Blinken, called it a “difficult decision”, but claimed he had to act because “the Ukrainians are running out of ammunition”.
However, Mr Sunak publicly refused to back the move while speaking to reporters on Saturday, and pointed out that the UK is a “signatory to a convention which prohibits the production or use of cluster munitions and discourages their use”.
He added: “We will continue to do our part to support Ukraine against Russia’s illegal and unprovoked invasion, but we’ve done that by providing heavy battle tanks and most recently long-range weapons, and hopefully all countries can continue to support Ukraine.”
Some of his Conservative colleagues have gone further, with Tory MP and Commons Defence Committee chairman Tobias Ellwood urging the US to “reconsider”.
He tweeted: “This is the wrong call and will alienate international good will. Their use leaves deadly unexploded ordnance over the battlefield, killing & injuring civilians.”
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Critics say cluster bombs, which are dropped across a wide area, kill and maim people indiscriminately.
They have also been compared to landmines because they can fail to detonate on impact and sometimes lay undiscovered for years – until unsuspecting civilians are blown up when they accidentally set them off.
The US’s Western allies appear split on the decision – Germany has declined to criticise the move, while Spain has expressed concerns.
Both Russia and Ukraine have already deployed cluster munitions so far in the war and, like the US, are not signatories to the international agreement banning their use.
Despite the tensions, the White House said Mr Biden was looking forward to the talks with Mr Sunak, which will include the pair “comparing notes” on Ukraine’s counteroffensive against Russia.
Mr Sunak and Mr Biden are not expected to hold a joint news conference in the UK.
Meanwhile, Downing Street has said Mr Sunak will use the NATO summit to urge allies to increase their military spending.
In a statement, the prime minister said: “As we face new and unprecedented challenges to our physical and economic security, our alliances are more important than ever.
“The UK is Europe’s leading NATO ally, we are the United States’ most important trade, defence and diplomatic partner, and we are at the forefront of providing Ukraine with the support they need to succeed on the battlefield.
“We have forged and invested in these alliances because we know they are the foundation of our strength and security.”
The UK is among a minority of NATO members that meets the alliance’s commitment to spend at least 2% of national income on defence.
France, Germany and Spain did not meet the target last year, according to estimates.
Mr Biden will finish his European tour with a visit to the Finnish capital Helsinki for a US-Nordic leaders summit, before flying back to the US on Thursday.