Russia is spying on vital UK infrastructure weak spots, and could kill "thousands and thousands and thousands" of people, UK defence secretary Gavin Williamson has warned.
A specific risk was four subsea electricity connections with the continent, and a further four subsea connections for gas imports, which he said Russia was probing.
He said it was "the real threat… the country is facing at the moment", in language the BBC said would be seen as "alarmist".
In an interview with newspaper The Daily Telegraph, Williamson said Moscow was spying on energy supplies which, if cut, could cause "total chaos" in the country.
His claim comes as the UK’s Ministry of Defence is trying to ward off spending cuts.
New in his post since November, Williamson said Russia had been researching these types of connections and was prepared to act in a way "any other nation would see as completely unacceptable".
He told the Telegraph: "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."
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 probing how to get into the UK’s critical infrastructure.
Earlier this week the chief of the National Cyber Security Centre, Ciaran Martin, said Russia had already staged attacks against Britain’s media, telecommunications and energy sectors over the past year, the BBC reported.
Image: Russian President Vladimir Putin en route to the Khmeimim air base in Syria, 11 December 2017 (Kremlin photograph)