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The interim report said that customers were overcharged £1.7bn a year between 2009 and 2013

Power supplies will be sufficient to keep the lights on this winter, the operator of Britain’s electricity transmission system has forecast, but only because of emergency measures introduced to avoid blackouts.

National Grid predicted the buffer between supply and demand during the winter months was likely to average 5.5 per cent — similar to last year’s margin — and said this was “manageable”.

However, the margin would have been just 0.1 per cent without spare capacity that power generators are paid to keep on standby, as well as a scheme to pay industrial users to reduce demand at peak times.

National Grid has handed lucrative contracts to several energy companies to keep mothballed coal-fired power stations available for use at short notice as Britain’s creaking electricity network has come under increasing strain.

Cordi O’Hara, director of UK market operations for National Grid, said she was confident “that we have the right tools and services available, including extra power we can call on if we need it, for times of highest demand”.

Fears of blackouts have been heightened by a series of headline-grabbing supply crunches caused by breakdowns and fires at UK power stations in recent years but Hugo Chandler, director of New Resource Partners, an energy consultancy, said National Grid deserved credit for making the system robust enough to cope.

“We are moving towards a decentralised, flexible grid with a diverse mix of generation capacity, and increasingly smart methods of balancing supply and demand,” he said.

However, in a longer-term outlook issued earlier this week, National Grid made clear that big new sources of capacity were needed in years ahead to keep the UK adequately supplied with electricity as dirty coal-fired plants are phased out to meet climate change targets.

Amber Rudd, energy secretary, wants to end UK coal power by 2025 — the same year that the first of a new generation of nuclear plants is scheduled to open at Hinkley Point in Somerset.

EDF of France, which is planning to build the Hinkley facility, said this week it remained committed to the project despite the UK’s vote to leave the EU but has yet to make a final investment decision amid concern among unions and other critics over the £18bn cost.

Until new nuclear arrives, the UK will become more dependent on inherently unreliable wind power as well as electricity imported from continental Europe by interconnectors. National Grid said demand management had an increasingly important role to play, with growing numbers of industrial users signing up to its scheme to incentivise lower power consumption at times of high demand.

“It’s very unlikely that the lights will go out this winter,” said Paul Massara, former chief executive of Npower, one of the UK’s “big six” electricity suppliers, and now head of North Star Solar, a green energy company. “But we’re dealing with a lot of old, creaking kit and the government needs to start taking strategic decisions if we’re to replace it in a sensible manner.”