Britain will immediately impose hard economic sanctions on Russia after President Vladimir Putin ordered the deployment of troops to two breakaway regions in eastern Ukraine, Prime Minister Boris Johnson said on Tuesday.

A Reuters witness saw tanks and other military hardware moving through the separatist-controlled city of Donetsk after Putin formally recognized the breakaway regions and ordered the deployment of Russian forces to "keep the peace."

"We will immediately institute a package of economic sanctions," Johnson told reporters.

"This is, I should stress, just the first barrage of U.K. economic sanctions against Russia, because we expect I'm afraid that there is more Russian irrational behaviour to come."

Johnson said he would set out the sanctions in the House of Commons, probably at around 12:30 GMT.

The sanctions, Johnson said, would be "targeted not just at entities in Donbass and Luhansk and Donetsk, but in Russia itself - targeting Russian economic interests as hard as we can."

Britain has threatened to cut off Russian companies' access to U.S. dollars and British pounds, blocking them from raising capital in London and to expose what Johnson calls the "Russian doll" of property and company ownership.

Russia's once mighty superpower economy is now smaller than Italy's based on IMF data, with a nominal GDP of around US$1.7 trillion.

HIT RUSSIA HARD

Britain has not yet spelled out who would fall under the sanctions, but has pledged that there would be nowhere for Russian oligarchs to hide. Johnson has said targets could include Russian banks.

"They will hit Russia very hard," Johnson said. "Be in no doubt that if Russian companies are prevented from raising capital on U.K. financial markets, if we unpeel the facade of Russian ownership of companies, of property, it will start to hurt."

Hundreds of billions of dollars have flowed into London and Britain's overseas territories from Russia since the fall of the Soviet Union in 1991, and London has become the Western city of choice for the super-wealthy of Russia and other former Soviet republics.

Putin will find he has "gravely miscalculated," Johnson said, adding that Moscow appeared to be bent on a full scale invasion of its former Soviet neighbour.

Johnson chaired a meeting of Britain's national emergency security committee early on Tuesday.

"I think that the tragedy of the present situation is that President Putin has surrounded himself with like-minded advisers who tell him that Ukraine is not a proper country. And I think that he is going to find that he has gravely miscalculated," Johnson told reporters after the meeting.

Russia's actions in Ukraine has created a situation as grave as the 1962 Cuban missile crisis when a confrontation between the United States and the Soviet Union brought the world to the brink of nuclear war, Johnson's health minister said.

"You can conclude that the invasion of Ukraine has begun," British Health Secretary Sajid Javid told Sky News. "The Russians, President Putin, has decided to attack the sovereignty of Ukraine and its territorial integrity."

The Cuban missile crisis erupted in 1962 when the Soviet Union responded to a U.S. missile deployment in Turkey by sending ballistic missiles to Cuba.

(Reporting by Kylie MacLellan; writing by Guy Faulconbridge; editing by Kate Holton and Jane Merriman)