In a highly unusual move based on declassified intelligence, the Foreign Office alleged that a former Ukrainian MP was "being considered as a potential candidate" as a new leader in what would be a significant escalation of the crisis.

But Yevhen Murayev, 45, who owns a TV channel and lost his parliamentary seat in 2019, dismissed the allegation as "nonsense".

"As someone who has been under Russian sanctions for four years, barred from Russia as a national security threat and whose father got his assets frozen in Russia, I find it hard to comment on the Foreign Office's statement," he told The Telegraph newspaper.

In a Facebook post on Sunday he said Ukraine needs new leaders.

"The time of pro-Western and pro-Russian politicians in Ukraine is gone forever," he wrote. "Ukraine needs new politicians whose policy will be based solely on the principles of the national interests of Ukraine and the Ukrainian people."

Whitehall sources said the Foreign Office claim was based on a UK intelligence analysis, which drew on various sources, including from the US. As a result, one of the Whitehall sources said that it was based on US-led intelligence.

Ukraine: UK will stand 'shoulder to shoulder' with Kyiv but 'extremely unlikely' to send troops

PM warns against naivety

President Volodymyr Zelenskiy is the current leader of Ukraine. He would like his country to join the European Union and the NATO defence alliance.

"We have information that indicates the Russian government is looking to install a pro-Russian leader in Kyiv as it considers whether to invade and occupy Ukraine," the Foreign, Commonwealth and Development Office said in a brief statement. It did not elaborate further on how this plan would be achieved.

The claims came as Boris Johnson cautioned against being "naive" when dealing with President Vladimir Putin following weeks of dialogue between Russia and Western allies to try to resolve the crisis over Ukraine and wider European security.

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"The Prime Minister's view is that the situation in Ukraine is the biggest test to the unity and resolve of the West and the NATO alliance in decades," a Downing Street source said.

"It would be frankly naive to assume that Russia could be mollified by changes to the European security architecture - the Kremlin's behaviour has made that consistently clear."

The source said Mr Johnson regards the stand-off as "gravely dangerous" and is being briefed on the situation daily.

He is expected to tell Ben Wallace, the defence secretary, to accept an invitation to meet with the Russian defence minister in Moscow in the coming days.

Russian activity "subverts" Ukraine

As well as warning of a de facto coup plot, the Foreign Office claimed to have information that Russia's intelligence services maintain links with "numerous" former Ukrainian politicians, including ex-prime ministers and chief of staff to former president Viktor Yanukovych.

"Some of these have contact with Russian intelligence officers currently involved in the planning for an attack on Ukraine," it said.

The Foreign Office did not provide evidence to support the allegations given the secretive nature of its sources.

Liz Truss, the foreign secretary, said: "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."

Ms Truss added in the statement: "Russia must de-escalate, end its campaigns of aggression and disinformation, and pursue a path of diplomacy. As the UK and our partners have said repeatedly, any Russian military incursion into Ukraine would be a massive strategic mistake with severe costs."

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Conflict playing out in the 'grey zone' of warfare

Information is being used as a tactic by both sides to undermine the other or gain an advantage as tensions between Russia and Western allies mount over Ukraine.

It is a form of virtual conflict that takes place in a grey zone under the threshold of war.

The suggestion of Mr Murayev as the potential head of a new puppet regime to Moscow was met with scepticism by some analysts.

Mark Galeotti described him as a "pretty marginal figure". He pointed out that Mr Murayev is opposed to fellow politician Viktor Medvedchuk, who is considered to be Russian President Vladimir Putin's closest ally in Ukraine.

The foreign office statement, despite apparently being based on US-led intelligence, also identified largely different individuals than a similar statement by the US government that was published last week and also accused Russia of influence operations in Ukraine.

"The 'there's a coup brewing' narrative depressingly fits neatly with the 'Russia's about the invade' one, and also a maximalist sense of what 'invasion' might mean - not some further militarisation of the Donbas but a full-on blitzkrieg to take the whole country," Mr Galeotti wrote on Twitter.

"Such a massively ambitions objective seems truly implausible - and it is worth noting that even Kyiv seems to treat it as such. Of course that doesn't preclude lunacy in the Kremlin, but we ought to treat it as an outlier of a threat."

PM expected to increase pressure on Kremlin

The Foreign Office also named former Ukrainian politicians that Russian spies allegedly maintained contact with as Serhiy Arbuzov, an acting prime minister in 2014, Mykola Azarov, a former prime minister, Andriy Kluyev, chief of staff to Mr Yanukovich and Vladimir Sivkovich, a former deputy head of the Ukrainian National Security and Defence Council.

Mr Sivkovich was called out earlier in the week by the US government, which also accused the Russian government of influence operations to destabilise Ukraine.

The UK allegations emerged as Mr Johnson was expected to increase pressure on Russia this week, with Whitehall departments preparing to stand-up specialist counter-disinformation cells.