The cost of ensuring Britain could turn its lights back on after a catastrophic nationwide blackout has soared by at least £12m this year, as National Grid is forced to pay struggling old coal plants to “keep warm” in case of an emergency.

Britain’s so-called “black start” plans are designed to ensure that electricity supplies could be swiftly restored in the event of an unprecedented power failure plunging all or part of the country into darkness.

As most power plants need to draw some electricity from the grid to start generating, National Grid has to ensure the UK retains a certain number of black start plants that are able to fire up independently using their own generators.

Historically, several of the UK’s coal plants have been relied upon to form part of the black start plan.

But rising green taxes, cheap gas prices and the growth of renewables are together rendering the coal plants increasingly uneconomic, with some closing down for good and most others now only running for parts of the day.

This poses a threat to Britain’s emergency plans because if the plants are not generating when a catastrophic power failure hits, they will take far longer to start up.