British Prime Minister Keir Starmer on Monday said he would not "get into operational details" after US President Joe Biden gave Ukraine permission to use Western-supplied long-range missiles against Russia.
Speaking to broadcasters at the G20 in Brazil, Starmer refused to be drawn "because the only winner, if we were to do that, is (Russian President Vladimir) Putin".
Kyiv has long sought authorisation from Washington to use the powerful Army Tactical Missile System, or ATACMS, to hit military installations inside Russia as its troops face growing pressure.
A US official said Washington's major policy shift on the missiles was in response to Russia's deployment of thousands of North Korean troops in its war effort.
Britain, which has provided Ukraine with Storm Shadow long-range missiles, has consistently pushed to ease restrictions on Kyiv's use of the weapons.
Putin had previously warned that letting Ukraine use long-range weapons would mean NATO was "at war" with Moscow.
In parliament in London, lawmaker Roger Gale asked if Britain planned to "align with the United States" in granting Kyiv permission to use the UK-supplied missiles "as it sees fit in its own defence".
Junior defence minister Maria Eagle said the government intended to "align with our allies" on how Ukraine "can make use of the capabilities that's been offered" by its backers.
Starmer added: "I've been really clear for a long time now we need to double down.
"We need to make sure Ukraine has what is necessary for as long as necessary, because we cannot allow Putin to win this war," he said.
Asked if he had spoken to Russian Foreign Minister Sergei Lavrov at the G20, he said: "I haven't spoken to Russia and I've got no plans to do so."
Foreign Secretary David Lammy, speaking to reporters after a UN Security Council meeting in New York, also refused to discuss the use of British missiles, because it "risks operational security".
Asked how concerned he was about the implications of Donald Trump's presidency on the war in Ukraine, he said: "One president at a time."
"We're dealing with President (Joe) Biden and we are committed to putting Ukraine in the strongest possible position," he added.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
