Russia could cause "thousands and thousands and thousands of deaths" by crippling UK infrastructure, the defence secretary has warned.
Gavin Williamson told the Daily Telegraph that Moscow was spying on energy supplies which, if cut, could cause "total chaos" in the country.
He said it was "the real threat... the country is facing at the moment."
Head of the British army, Sir Nick Carter, has warned the UK is struggling to keep up with Russian capabilities.
The UK has four undersea connections for electricity linked to the continent and a further four for gas to bring energy into the country.
Mr Williamson, who became defence secretary last November, said Russia had been researching these types of connections and would be willing to take action "any other nation would see as completely unacceptable".
He told the paper: "The plan for the Russians won't be for landing craft to appear in the South Bay in Scarborough, and off Brighton beach.
"They are going to be thinking, 'how can we just cause so much pain to Britain?
"Damage its economy, rip its infrastructure apart, actually cause thousands and thousands and thousands of deaths, but actually have an element of creating total chaos within the country."
Official representative of the Russian Defence, Ministry Major-General Igor Konashenkov, accused the defence secretary of losing "his understanding of where the limits of common sense lie long ago".
He added that Mr Williamson's "morbid fear" of Russian activities belonged in a children's comic book or an episode of Monty Python's Flying Circus.
The BBC's defence correspondent, Jonathan Beale, said some may see the defence secretary's language as "alarmist".
But he was backed by former First Sea Lord and security minister Lord West, who told the paper he was "absolutely certain" Russia was looking at how to get into the UK's critical infrastructure.
The warning comes after the chief of the National Cyber Security Centre, Ciaran Martin, said earlier this week that Russia had already staged attacks against Britain's media, telecommunications and energy sectors over the past year.
It comes as the Ministry of Defence is under pressure to avoid cuts that could be coming from the Treasury.
Mr Williamson has already been told there could be a Tory revolt over reductions to army numbers and naval capability, and suggested he will be asking the chancellor for more money.