Britain made the accusation late on Saturday, also saying Russian intelligence officers had been in contact with a number of former Ukrainian politicians as part of plans for an invasion.

The Russian Foreign Ministry dismissed the comments as "disinformation", accusing Britain and the transatlantic military alliance NATO of "escalating tensions" over Ukraine.

The British accusations, first made in a statement by the foreign ministry, come at a time of high tensions between Russia and the West over Russia's massing of troops near its border with Ukraine.

The foreign ministry said it had information the Russian government was considering former Ukrainian lawmaker Yevhen Murayev as a potential candidate to head a pro-Russian leadership.

The British foreign ministry declined to provide evidence to back its accusations.

A ministry source said it was not usual practice to share intelligence matters, and the details had only been declassified after careful consideration to deter Russian aggression.

Britain, which this week supplied 2,000 missiles and a team of military trainers to Ukraine, also said it had information that Russian intelligence services were maintaining links with numerous former Ukrainian politicians, including senior figures with links to ex-President Viktor Yanukovich