The UK has accused President Putin of plotting to install a pro-Moscow figure to lead Ukraine's government, BBC reports.
The Foreign Office took the unusual step of naming former Ukrainian MP Yevhen Murayev as a potential Kremlin candidate.
Russia has moved 100,000 troops near its border with Ukraine but denies it is planning an invasion.
UK ministers have warned that the Russian government will face serious consequences if there is an incursion.
In a statement, Foreign Secretary Liz Truss 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.
"Russia must de-escalate, end its campaigns of aggression and disinformation, and pursue a path of diplomacy."
The Russian Ministry of Foreign Affairs tweeted that the Foreign Office was "circulating disinformation" and urged it to "cease these provocative activities" and "stop spreading nonsense".
Deputy Prime Minister Dominic Raab said there was "a very serious risk" of invasion but there would be "severe economic consequences", including sanctions, if Russia took that step.
However, he told the BBC's Sunday Morning programme it was "extremely unlikely" British troops would be sent to defend Ukraine, adding that the country was not a Nato ally.
Asked if the threat of sanctions would be enough to deter Russia from invading, Raab said Vladimir Putin would also be worried about getting "bogged down in Ukraine" and ending up "with another Chechnya", where there was several years of conflict in the 1990s.
He added that Russia "needs to live up to the basic tenets of international law and invading another country is not one of those".
Russia has denied it is planning any attack but Putin has issued demands to the West, including that Ukraine be prevented from joining the Nato military alliance.
He also wants Nato to abandon military exercises and stop sending weapons to eastern Europe, seeing this as a direct threat to Russia's security.
Russia has seized Ukrainian territory before, when it annexed Crimea in 2014, after the country overthrew their pro-Moscow president.
Ever since, Ukraine's military has been locked in a war with Russian-backed rebels in areas of the east near Russia's borders.
