Donald Trump was warned by Britain that it was in America’s interest that Vladimir Putin is not allowed to win the Ukraine war.
Defence Secretary John Healey laid out the argument ahead of Ukraine’s president Volodymyr Zelensky addressing the Cabinet in Downing Street.
He was also to hold a bilateral meeting with Sir Keir Starmer in No10.
The meetings in Whitehall come just hours after Trump claimed he could end the war in Ukraine “with a telephone call,” fuelling fears that if elected for a second term, he would seek to force Kyiv to the negotiating table even if it meant handing over swathes of land to Russia.
Mr Healey told Times Radio, clearly referring to China’s threat to Taiwan: “For any president whose first interest is America’s interests then the argument to them is that if Putin prevails in Ukraine, Europe is less secure and it’s directly in America’s interests that big, aggressive, autocratic states do not see that they can get away with redrawing sovereign boundaries by force in the way that Russia is trying to do.”
He later told ITV’s Good Morning Britain: “It’s the Ukrainians who are fighting. It’s their call when they stop.”
Mr Zelensky is pushing for more freedom for his armed forces to use weapons supplied by the West to strike deep inside Russia at sites from where missiles are being launched into his country.
The UK is providing missiles and other equipment to Kyiv for Ukraine’s defence and this does not “preclude” hitting targets inside Russia.
But Mr Healey said the details of consents for the use of British missiles, which also include components from at least one other country, were “complex”.
But he made clear that “intense” discussions were taking place with Ukraine’s government on whether the restrictions could be relaxed.
Russian forces, though, have been gaining more ground in eastern Ukraine, with troops being thrown into battles and suffering high casualties, according to British defence chiefs.
Mr Zelensky was set to be the first official visitor to Downing Street under Sir Keir's premiership, and the first foreign leader to address the Cabinet in person since US president Bill Clinton in 1997.
He is expected to brief ministers about the situation in Ukraine and the need to expand Europe's defence industrial base, as well as agreeing a £3.5 billion defence export finance deal with Sir Keir.
The Prime Minister said: "Ukraine is, and always will be, at the heart of this Government's agenda and so it is only fitting that President Zelensky will make a historic address to my Cabinet.
"Russia's incremental gains on the battlefield are nothing compared with the collective international support for Ukraine, or the strength of ties between our people."
Speaking to the BBC ahead of the meeting, Mr Zelensky said he needed "strong support" from Sir Keir and would push for permission to use British long-range missiles against targets in Russia, particularly locations from which Russian forces were attacking civilian targets in Ukraine.
Mr Zelensky's visit to Downing Street follows the European Political Community (EPC) summit at Blenheim Palace, Oxfordshire, on Thursday, at which 44 countries and the EU agreed to target Russia's "shadow fleet" of tankers seeking to evade sanctions on Russian oil.
The group of around 600 vessels carries 1.7 million barrels of oil per day, providing funds for Moscow's war effort while also allegedly acting as listening stations or transporting weapons for Russia.
The ships are often old and unsafe, and engage in dangerous practices such as turning off their location tracking systems, increasing the risk of a serious collision.
Sir Keir said: "Alongside our European partners, we have sent a clear message to those enabling Putin's attempts to evade sanctions: we will not allow Russia's shadow fleet, and the dirty money it generates, to flow freely through European waters and put our security at risk."
Speaking at the EPC summit on Thursday, Mr Zelensky said the UK had been "one step ahead in its determination to support Ukraine" since the Russian invasion began.
He added: "This is the kind of resolve we need to stop Russian terror."
On Thursday, the UK also sanctioned 11 oil tankers thought to be involved in evading sanctions, including the Rocky Runner, which previously sought to escape UK action by changing its operator.
Most of the ships sail under the flag of Gabon, although some - including the Rocky Runner - are flagged with the Cook Islands and one sails under the flag of Panama.
Sanctioned vessels are prohibited from entering UK ports and may be detained or directed to enter a port.
