Great Britain faces its greatest risk of blackouts for six years this winter as old coal plants and nuclear reactors shut down and energy demand rises as the economy emerges from Covid-19 restrictions.

National Grid’s electricity system operator, which is responsible for keeping the lights on, said it expected the country’s demand for electricity to return to normal levels this winter, and would be braced for “some tight periods”.

The system operator published a surprise report warning that the tight electricity supplies recorded last winter could be tighter in the winter ahead due to “uncertainty” over the country’s power supplies.

It said that in some scenarios the “margin” of forecast electricity supplies might exceed demand by 5.3%, the tightest margin recorded since the winter of 2015-2016, when National Grid was forced to ask businesses to reduce their electricity usage to keep the lights on after a spate of breakdowns at coal plants.

National Grid has traditionally published its forecasts for the winter in September but surprised the market on Thursday by issuing a preview report.

“Following tighter margins in winter 2020-21 compared to previous winters, we have decided to publish an early view of the margin for winter 2021-22. We believe this will help to inform the electricity industry and support preparations for the winter ahead,” it said.

The system operator issued a string of official warnings that electricity supplies were under pressure last year, despite a 3-4% slump in energy demand as people stayed away from offices, pubs and restaurants during the Covid-19 pandemic.

The UK was forced to rely on its last remaining coal power plants to meet demand during cold, still periods when demand was high and wind speeds low last winter. This year it could be more difficult to cover the loss of a sudden outage at a power plant, subsea power cable or low wind speeds due to the closure of older nuclear plants.

The system operator has assumed that the Dungeness B and Hunterston B nuclear power stations will not be available for the full winter, and that the Baglan Bay, Severn Power and Sutton Bridge gas power stations will remain unavailable.

“While we remain confident there is sufficient supply to meet peak demand, we should prepare for some tight periods during the winter” because there “is still some uncertainty” about electricity supplies, the electricity system operator said.