Britain on Saturday alleged that it had information that Moscow was "looking to install a pro-Russian leader in Kiev" as fears of a Russian invasion of Ukraine grow.
Tensions have soared in recent weeks as tens of thousands of Russian troops mass on Ukraine's border, along with an arsenal of tanks, fighting vehicles, artillery and missiles.
London said it had seen evidence that several former Ukrainian politicians had maintained links with Russian intelligence services, and that former MP Yevgen Murayev was being considered as a potential leader.
Some of those in contact with Russian intelligence officers were "currently involved in the planning for an attack on Ukraine", the Foreign Office said in a statement, though did not release details of the evidence. A US official called the alleged plot "deeply concerning".
Moscow dismissed the claims as "disinformation", and urged London to "stop spreading nonsense".
"Disinformation circulated by @FCDOGovUK is yet another indication that it is the @Nato Members led by the Anglo-Saxon nations who are escalating tensions around #Ukraine," Russia's foreign ministry said in a tweet.
The accusations come at the end of a week of intense international diplomacy, which concluded with Antony Blinken and Sergei Lavrov, Washington and Moscow's top diplomats, agreeing to keep working to ease tensions.
Murayev, the man named by London, lost his seat in the Ukrainian parliament when his party failed to win five percent of the vote in 2019 elections.
He is considered to be an owner of Ukrainian TV station "Nash", which regulators have been seeking to shut down since last year, accusing it of airing pro-Russian propaganda.
The other four politicians named by the UK were Mykola Azarov, Sergiy Arbuzov, Andriy Kluyev and Volodymyr Sivkovich.
Azarov served as prime minister under pro-Moscow president Viktor Yanukovych. Both fled Kiev for Russia following the 2014 uprising in Ukraine that toppled a government that had rejected pressure to move the country closer to the West.
Sivkovich, a former deputy secretary of the Ukrainian National Security and Defence Council, was sanctioned by the United States this week for allegedly working with Russian intelligence.
Arbuzov and Kluyev both served as deputy prime minister under Yanukovych. (AFP)