Emergency supplies of electricity are being sought by the National Grid for this winter because of the threat of shortages of output from the UK's coal, gas and nuclear power stations.
National Grid said on Tuesday it was extending its search for additional sources of temporary supplies, blaming emergency shutdowns at two nuclear power stations operated by EDF of France and unexpected fires at two key coal-fired stations during recent months - Ironbridge in Shropshire and Ferrybridge in Yorkshire.
The move follows an admission by EDF last month that the detection of a fault in a boiler, which has prompted wider inspection work and output restriction at nuclear power stations at Heysham 1 and Hartlepool, would result in a substantial loss in supply this year.
The nuclear stations command a combined generating capacity of more than 2.3 gigawatts, while the UK's total generating capacity ended 2013 4.4GW lower at 85GW, as the closure of several stations was only partially offset by new supplies of renewable energy.
National Grid's previously "modest" assessment of reserve capacity need this winter has also been undermined by damage this year to 0.37GW of capacity at the Ironbridge coal-fired power station operated by Eon, which is too expensive to repair ahead of a planned shutdown of the site by 2015.
A subsequent fire at Ferrybridge power station, owned by SSE, in July also compromised output from two units at one of the UK's biggest generation sites capable of generating close to 1GW at peak load.
The announcement in July of the planned closure of the gas-fired Barking power station in east London has also prompted National Grid and the Department of Energy and Climate Change to take a more cautious short-term view towards the growing squeeze on the UK's power generation fleet.
National Grid, which had previously described the need for back-up generating capacity this winter as "modest", said on Tuesday there was "an increased level of uncertainty over the volume of plant that may be available in the market this winter".
Cordi O'Hara, National Grid's director of UK market operation, said an additional tender would be run to extend the so-called reserve capacity - power supplies that can be made available a short notice to meet peaks in demand - on top of a pilot auction this summer.
"This is a sensible precaution to take while the picture for this winter remains uncertain," Mr O'Hara said. "At this stage we don't know if these reserve services will be needed, but they could provide an additional safeguard."
National Grid, in plans approved by DECC and energy regulator Ofgem, has already set out plans to expand the programme of procurement to deal with the growing threat to stretched UK electricity supplies during the next four winters.
It is aiming to procure an additional 1.8GW of additional generating capacity in the winter after next to deal with the problem caused by the retirement of conventional fossil-fuelled stations to deal with the unpredictability of growing wind power supplies and slow progress in commissioning nuclear power stations.
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