Britain has accused President Vladimir Putin of plotting to install a pro-Moscow leader as head of the government in Ukraine as he considers an invasion against Russia's neighbour.
The Foreign Office took the unusual step of naming former Ukrainian MP Yevhen Murayev as a potential Kremlin candidate to take over in Kyiv.
The move comes as Western allies stepped up warnings Russia will pay a heavy price if the estimated 100,000 troops massed on the border launch any kind of incursion into Ukraine.
In a statement, Foreign Secretary Liz Truss said the Russian plotting showed the lengths to which the Kremlin was prepared to go to undermine the government in Kyiv.
"Russia must de-escalate, end its campaigns of aggression and disinformation, and pursue a path of diplomacy," Ms Truss said.
"As the UK and our partners have said repeatedly, any Russian military incursion into Ukraine would be a massive strategic mistake with severe costs."
Watch: Which UK anti-tank missile is Ukraine getting as Russia tension rises?
Mr Murayev, a media owner, lost his seat in the Ukrainian parliament when his party failed to secure 5% of the vote in elections in 2019.
He has reportedly spoken out in the past in support of Russia's annexation of Crimea from Ukraine in 2014.
In addition, the Foreign Office named four other Ukrainian politicians who, it said, maintained links with the Russian intelligence services.
It said some of them had been in contact with Russian intelligence officers working on the invasion plan.
They include Mykola Azarov who served as prime minister under the pro-Moscow president Viktor Yanukovych who was toppled in a popular uprising in 2014.
Watch: Defence Secretary confirms 'light anti-armour defensive weapons' for Ukraine amid Russia tensions.
Mr Azarov fled to Russia where he established what was widely seen as a puppet government-in-exile.
He has been the subject of international sanctions and an Interpol "red notice" issued at the request of the Ukrainian government.
Russia's Foreign Ministry rejected the UK's claim it was trying to install a pro-Moscow leader in Ukraine.
"The disinformation spread by the British Foreign Office is more evidence that it is the NATO countries, led by the Anglo-Saxons, who are escalating tensions around Ukraine," Russian Foreign Ministry spokeswoman Maria Zakharova said.
"We call on the British Foreign Office to stop provocative activities, stop spreading nonsense."