President of Russia, Vladimir Putin (Getty)

Sir Adrian told the Royal United Services Institute there was a danger such a campaign of undercover attacks could paralyse Nato decision making, as members disagreed over how much Russia was responsible, and how to respond.

Nato commanders fear a campaign of skilfully disguised, irregular military action by Russia, which is carefully designed not to trigger the alliance's mutual defence pact.

He said the "resulting ambiguity" would make "collective decisions relating to the appropriate responses more difficult".

But Sir Adrian, one of the most senior generals in the British Army and a former director of special forces, went further and said there was also danger that Russia could use conventional forces and Soviet-era brinkmanship to seize Nato territory.

He said Russia had shown last year it could generate large conventional forces at short notice for snap exercises along its borders. There was a danger these could be used not only for intimidation and coercion but potentially to seize Nato territory, after which the threat of escalation might be used to prevent re-establishment of territorial integrity. This use of so called escalation dominance was of course a classic Soviet technique.

He went on to say that the threat from Russia, together with the risk it brings of a miscalculation resulting in a strategic conflict, however unlikely we see it as being right now, represents an existential threat to our whole being.

Defence Secretary Michael Fallon (PA)

Nato has agreed to set up a rapid reaction force of around 5,000 troops ready to move at 48 hours notice, in case of Russian agression in Eastern Europe. Supplies, equipment and ammunition will be stockpiled in bases in the region. Alliance leaders hope the force will deter any incursion.

David Cameron warned Vladimir Putin there will be more sanctions and "more consequences" for Russia if the ceasefire in Ukraine does not hold.

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