Moscow on Sunday rejected Britain's accusation that the Kremlin is seeking to install a pro-Russian leader in Ukraine.
Britain said earlier that Russian intelligence officers were in contact with a number of former Ukrainian politicians as part of plans for an invasion.
Britain's Foreign Office declined to provide evidence to back its accusations, which came at a time of high tensions between Russia and the West over Russia's massing of troops near its border with Ukraine. Moscow has insisted it has no plans to invade.
The Foreign Office said it had information that the Russian government was considering former Ukrainian lawmaker Yevhen Murayev as a potential candidate to head a pro-Russian leadership.
"We will not tolerate Kremlin plot to install pro-Russian leadership in Ukraine," British Foreign Secretary Liz Truss said on Twitter. "The Kremlin knows a military incursion would be a massive strategic mistake & the UK and our partners would impose a severe cost on Russia."
The British statement was released in the early hours of Sunday, Moscow and Kyiv time.
A Foreign Office source said it was not usual practice to share intelligence matters and that the details had only been declassified after careful consideration to deter Russian aggression.
Accusations are 'nonsense': Russian ministry
Russia's Foreign Affairs Ministry dismissed the comments as "disinformation," accusing Britain and NATO of "escalating tensions" over Ukraine.
"We urge the Foreign Office to cease these provocative activities, stop spreading nonsense and finally concentrate its efforts on studying the history of the Mongol-Tatar yoke," the ministry said on its verified Facebook account.
The British claims come a day after the top U.S. and Russian diplomats failed to make a major breakthrough in talks to resolve the crisis over Ukraine, although they agreed to keep talking.
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Russia has made security demands on the United States, including a halt to NATO's eastward expansion and a pledge that Ukraine will never be allowed to join the Western military alliance.
U.S. National Security Council spokesperson Emily Horne said in a statement: "This kind of plotting is deeply concerning. The Ukrainian people have the sovereign right to determine their own future, and we stand with our democratically elected partners in Ukraine."
'Contact with Russian intelligence officers'
Murayev, 45, is a pro-Russian politician who opposes Ukraine's integration with the West. According to a poll by the Razumkov Centre think-tank conducted in December 2021, he was ranked seventh among candidates for the 2024 presidential election with 6.3 per cent support.
"You've made my evening. The British Foreign Office seems confused," Murayev told Britain's Observer newspaper. "It isn't very logical. I'm banned from Russia. Not only that but money from my father's firm there has been confiscated."
Britain, which this week supplied 2,000 missiles and a team of military trainers to Ukraine, also said it had information that Russian intelligence services were maixjmtzywntaining links with "numerous" former Ukrainian politicians, including senior figures with links to ex-president Viktor Yanukovych.
Yanukovych fled to Russia in 2014 after three months of protests against his rule and was sentenced in absentia to 13 years in prison on treason charges in 2019.
"Some of these have contact with Russian intelligence officers currently involved in the planning for an attack on Ukraine," the British Foreign Office statement said.
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Prime Minister Boris Johnson's Downing Street office also said he was planning to ramp up pressure on Russia this week by calling for European counterparts to come together with the U.S. to face down Russian aggression.
Earlier, RIA news agency reported that Truss would visit Moscow in February to meet her Russian counterpart, Sergey Lavrov, while Russian Defence Minister Sergei Shoigu and his British counterpart, Ben Wallace, have also agreed to hold talks.
