The government is testing plans to cope with energy blackouts that could last up to seven days, according to a report.
A series of exercises has been held with government departments and councils across the country in recent days, according to The Guardian.
Programme Yarrow was drawn up in 2021 to improve planning and resilience in the event of a major technical fault on the National Grid - such as flood damage, a lightning strike on a substation or an attack by a hostile state on sub-sea power cables.
The programme's worst-case scenario is that all sectors - including transport, food, water supply, communications and energy - are hit for up to a week.
All premises without backup generators would have their power cut without warning, with 60% of electricity demand being met between day two and day seven, when houses and businesses would be given "intermittent access" to ration supply, the report says.
In this scenario, only analogue FM radios would work, with only BBC Radio 2 and 4 likely to be broadcasting.
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An agreement between energy regulator Ofgem and National Grid says that 100% of electricity demand should be restored after a week, and the government expects that target to be met even in a worst-case scenario, the newspaper reported.
The programme is aimed at a more severe situation than that outlined by National Grid last month, which warned of rolling three-hour blackouts under a worst-case scenario this winter.
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A source told The Guardian: "The government doesn't want any publicity on Yarrow, as they don't want it to be seen as linked to Ukraine, energy supply and the cost of living.
"But we need to think about how we can help people in advance.
"The fact they're talking about it now means they have a real concern it could happen."
The government told Sky News: "As a responsible government, it is right that we plan for all potential scenarios and work with industry to prepare and exercise robust contingency plans.
"This work is ongoing continuously and is an important strand of our national resilience planning.
"Local and national exercises are a part of this ongoing work and ensure we are able to effectively respond to any of a wide range of scenarios, no matter how unlikely they may be."
The programme predates the war in Ukraine but concern has grown with Russian threats to cut off energy supply to Europe.