The Sellafield nuclear power plant is among facilities told to prepare for a Kremlin-sponsored assault

Spy chiefs have warned the bosses of Britain’s key power companies to boost their security amid fears of a Russian cyber-attack that could put the lights out.

The National Grid was put on alert last week by officials from the National Cyber Security Centre (NCSC) — a branch of the signals intelligence agency GCHQ — and given advice on how to improve its defences to prevent power cuts.

Electricity, gas and water firms, the Sellafield nuclear power plant, Whitehall departments and NHS hospitals have all been warned to prepare for a state- sponsored assault ordered by the Kremlin after the nerve agent attack in Salisbury.

NCSC officials, working with the National Crime Agency and MI5’s Centre for the Protection of National Infrastructure, have told