Boris Johnson has issued a direct warning to President Vladimir Putin there will be "significant consequences" for Russia if it invades neighbouring Ukraine.
The Prime Minister spoke by telephone to Mr Putin on Monday to reassert the UK’s commitment to Ukraine’s territorial integrity and warn that any destabilising action by Moscow would be a "strategic mistake".
Mr Johnson’s call marks the latest intervention by Western leaders urging the Kremlin to draw back amid growing alarm at the build-up of Russian forces in the region.
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A No 10 spokesman said: "He expressed the United Kingdom’s deep concern over the build-up of Russian forces on Ukraine’s border, and reiterated the importance of working through diplomatic channels to de-escalate tensions and identify durable solutions.
"The Prime Minister emphasised the UK’s commitment to Ukraine’s territorial integrity and sovereignty, and warned that any destabilising action would be a strategic mistake that would have significant consequences."
On Sunday, G7 foreign ministers meeting in Liverpool issued a statement reminding Moscow that "any use of force to change borders is strictly prohibited under international law" and pledging to inflict "severe cost" on Russia if it does mobilise against its neighbour.
On Monday, the European Union announced that it was imposing sanctions on the Russian Wagner Group of military contractors which has been accused of seeking to destabilise Ukraine.
According to US intelligence, Russia has stationed about 70,000 troops near the border Ukraine and has begun planning for a possible invasion as soon as early next year.
Moscow has denied that it is preparing an invasion and has accused the government in Kiev of stoking tensions in the region by deploying new weapons.
The two countries have been at odds since 2014 when Russian forces annexed the Crimean peninsula and backed a separatist rebellion in eastern Ukraine.
Last week US President Joe Biden spoke to Mr Putin to warn him of "economic consequences like you’ve never seen" if he was to mount a fresh incursion.
The Russian president has in turn been pressing for assurances that NATO will not be expanded to include Ukraine.