Britain made the accusation late on Saturday, also saying Russian intelligence officers had been in contact with a number of former Ukrainian politicians as part of plans for an invasion.
Russia's foreign ministry dismissed the comments as "disinformation", accusing Britain and NATO of "escalating tensions" over Ukraine.
"There'll be very serious consequences if Russia takes this move to try and invade but also install a puppet regime," Deputy Prime Minister Dominic Raab told Sky News on Sunday.
The accusations come 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.
In a statement late on Saturday, the British foreign ministry declined to provide evidence to back its accusations.
It said it had information the Russian government was considering former Ukrainian MP 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.
A foreign ministry source said it was not usual practice to share intelligence matters, and the details had only been declassified after careful consideration to deter Russian aggression.
Murayev, 45, is a pro-Russian politician who opposes Ukraine's integration with the West. According to a poll by the Razumkov's 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."
The British claims came a day after the top US and Russian diplomats failed to make a major breakthrough in talks to resolve the crisis over Ukraine, although they agreed to keep talking.
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.
US 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."
Britain, which this week supplied 2000 missiles and a team of military trainers to Ukraine, also said it had information that Russian intelligence services were maintaining links with "numerous" former Ukrainian politicians, including senior figures with links to ex-President Viktor Yanukovich.
Yanukovich fled to Russia in 2014 after three months of protests against his rule and was sentenced in absentia to 13 years in jail on treason charges in 2019.
