Britain has accused the Kremlin of seeking to install a pro-Russian leader in Ukraine.
The UK's foreign ministry declined to provide evidence to back up its claims, which come at a time of heightened tensions between Russia and the West.
Russia has massed troops at its border with Ukraine, but insists it has no plans to invade.
In a statement Britain's Foreign Office said it had information that the Russian government was eying former Ukrainian lawmaker Yevhen Murayev to potentially head a pro-Russian leadership.
Murayev, who opposes Ukraine's integration with the West, described the British Foreign Office as seeming "confused".
He told the Observer newspaper that the statement "isn't very logical", adding that he is "banned from Russia".
The Russian Foreign Ministry dismissed the comments as "disinformation" and accused Britain and NATO of "escalating tensions".
The UK also said it had information that Russian intelligence services were maintaining links with "numerous" former Ukrainian politicians - including senior figures linked to ex-President Viktor Yanukovich who fled to Russia in 2014 after three months of protests.
The British claims came a day after top U.S. and Russian diplomats failed to make a breakthrough in discussions to resolve the crisis.
They have agreed to keep talking.
Russia has made demands on the U.S. including a halt to NATO's eastward expansion and a pledge that Ukraine will never be allowed to join the Western military alliance.
A UK foreign ministry source said it was not usual practice to share intelligence matters and that the details had only been declassified after careful consideration, in order to deter Russian aggression.
The office of UK Prime Minister Boris Johnson also said he was planning to ramp up pressure on Russia this week by calling for European counterparts to come together with the United States.