Ministers have been urged to stop "dithering" over plans for new nuclear stations as it emerged that nearly half of existing stations are out of action.
The government has said nuclear should be part of the energy 'mix'
British Energy has closed two of its 16 nuclear reactors for tests and another five are shut for maintenance.
Shadow business secretary Alan Duncan said government "dithering and delay". was putting energy security at risk.
But Business Secretary John Hutton has said a "quick decision" is due on the future of nuclear power.
British Energy shut two stations after discovering a corroded steel wire in the concrete casing of one of its reactors at Hartlepool power station. That and a similarly designed station, Heysham-1, near Morecombe, have been shut for tests.
Another five were already closed for maintenance.
Mr Duncan said British Energy had given a "disturbing revelation" adding: "Last year we were told that urgent decisions need to be taken about the future of our energy supplies, but all we have seen from the government is dithering and delay."
He said indecision was putting energy security at risk, by not giving the details necessary for decisions to be taken on nuclear, renewable energy or "carbon capture" technology.
The 16 nuclear stations run by British Energy are responsible for 18% of Britain's electricity.
Former environment minister Michael Meacher, an opponent of nuclear power, told BBC Radio 4's Today programme the number of offline power stations was "almost unprecedented", adding: "I think it is extremely worrying that one of the major sources of electricity is half down at this time."
He said there was a major problem due to breakdowns and the need for repair and maintenance.
"One needs certainty and the nuclear industry simply doesn't provide that," Mr Meacher said.
Most existing nuclear power stations are due to close by 2023 and the government has said its "preliminary view" is that new stations should be built as part of Britain's "energy mix" - to reduce carbon emissions and reliance on foreign oil and gas imports.
Professor Ian Fells, an adviser to the World Energy Council, said certainty was needed if private investors were to be persuaded to fund new nuclear power stations.
"What's lacking is any real decision on the part of the government and this dithering over nuclear power is probably the worst thing it can possibly do. It had better make its mind up pretty quick," he said.
John MacNamara, of the Nuclear Industry Association, said it was common for two or three stations to be shut down for maintenance or refuelling.
He said he thought there was enough capacity to ensure the lights would not go off, as a result of the closures, but added: "Ten years ago we had much more capacity.
"Nuclear power stations have been shutting down for some time, as have coal fired power stations. So things are a little tighter here, and I think that's why you're seeing the political urgency in this debate now"
He added: "We have got to get on and do something pretty quick."
Earlier this month, Mr Hutton told the Commons the government would make a "quick decision" on nuclear power, now its consultation period had ended.
But the Guardian newspaper reported on Tuesday that ministers were planning a U-turn on pledges to ensure that 20% of European energy comes from renewable sources, like wind and solar power, by 2020.
It reports that leaked documents show the commitment is expensive and faces "severe practical difficulties" and says Gordon Brown will be advised to persuade other governments to lower the target.