British Prime Minister Boris Johnson on Monday held talks with Russian President Vladimir Putin, telling him to back down with Ukraine, Downing Street said.
Mr Johnson echoed concern from the G7 on Sunday about the build-up of Russian troops on the border, urging a diplomatic solution to the stand-off, his office said.
His intervention comes after a two-day meeting of the G7 nations in which the grouping's top diplomats warned Moscow of "massive consequences" if it invades the former Soviet state.
Direct contact between London and Moscow has been rare since London expelled dozens of Russian diplomats after a nerve agent attack in the English city of Salisbury.
Britain blamed Russia for the 2018 attempted murder of former double agent Sergei Skripal and his daughter with the Cold War-era nerve agent Novichok.
It had already accused the Kremlin of being behind the radiation poisoning death of another former Russian agent, Alexander Litvinenko, in London in 2006.
Downing Street said Mr Johnson told Mr Putin of his "deep concern" at the Russian troop build-up on the border with Ukraine, which Moscow says is defensive.
The Kremlin said Mr Putin responded to Mr Johnson by condemning Kiev's "destructive line" and accusing Ukraine of "purposefully exacerbating the situation on the contact line" by using "heavy weapons and attack drones".
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He said the tensions were building as Nato countries were actively involved with Ukraine.
Mr Putin called for international agreements to stop the alliance advancing further east and sending weapons to neighbouring states to threaten Russia.
Mr Johnson repeated "the importance of working through diplomatic channels to de-escalate tensions and identify durable solutions", the UK government said.
"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," it said.
The G7, made up of Britain, Canada, France, Germany, Italy, Japan and the US, unanimously condemned Russia and warned it of severe repercussions.
All options, including wide-ranging political and economic sanctions, were on the table if Russia ignored a diplomatic solution, officials said.