Britain has alleged that it had information that Moscow was "looking to install a pro-Russian leader in Kyiv" 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.
Britain said on Saturday 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 United States 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.
Mr 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.
'Russia must de-escalate'
The other four politicians named by the United Kingdom were Mykola Azarov, Serhiy Arbuzov, Andriy Klyuyev and Vladimir Sivkovich.
Mr Azarov served as prime minister under pro-Moscow president Viktor Yanukovych. Both fled Kyiv for Russia following the 2014 uprising in Ukraine that toppled a government that had rejected pressure to move the country closer to the West.
Mr 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.
Mr Arbuzov and Mr Kluyev both served as deputy prime minister under Mr Yanukovych.
"The information being released today shines a light on the extent of Russian activity designed to subvert Ukraine, and is an insight into Kremlin thinking," said British Foreign Secretary Liz Truss.
"Russia must de-escalate, end its campaigns of aggression and disinformation, and pursue a path of diplomacy."
In Washington, US National Security Council spokeswoman Emily Horne said: "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."
The claims came hours after a senior UK defence source said that Russian Defence Minister Sergei Shoigu had accepted an invitation to meet British counterpart, Defence Secretary Ben Wallace, to discuss the crisis.
"Given the last defence bilateral between our two countries took place in London in 2013, Russian Defence Minister Sergei Shoigu has offered to meet in Moscow instead," the source said.
Ukraine 'will fight'
Few military experts believe that Kyiv's smaller forces - although rapidly modernising - could repel an outright Russian invasion.
But Ms Truss warned on Friday that Moscow still risked becoming embroiled in a "terrible quagmire" if it invaded.
"The Ukrainians will fight this," she warned.
Russia has put pressure on Ukraine since the 2014 uprising, Moscow seized the Crimean peninsula in 2014 and a few weeks later a pro-Russian insurgency broke out in eastern Ukraine that has since claimed more than 13,000 lives.
Britain is among a handful of Western nations rushing lethal weapons - such as anti-tank missiles - to Ukraine, dramatically increasing the prospect of Russian casualties.
Ukraine's Foreign Minister Dmytro Kuleba on Saturday condemned Germany for its refusal to supply weapons to Kyiv, urging Berlin to stop "undermining unity" and "encouraging Vladimir Putin".
The head of Germany's navy later resigned following controversial remarks on the crisis, a defence ministry spokesman said on Saturday.
Britain's Foreign Office on Saturday updated its travel advice to Ukraine in light of the ongoing crisis.
The department now advises against all travel to Donetsk oblast, Lugansk oblast and Crimea.
It also advises against all but essential travel to the rest of Ukraine and British nationals are advised to register their presence in the country.
