Demand for power from homes and businesses will exceed supply from the national grid within eight years, according to official figures.

The shortage of supplies will hit the equivalent of many as 16 million families for at least one hour during the year, it is forecast.

Not since the early 1970s when the three-day week was introduced to preserve coal has Britain faced the prospect of reationing energy use.

The gap between Britain's energy needs and demand throws fresh doubt on the Government's assertion that renewable energy can make up for dwindling nuclear and coal capabilities.

Over the next 10 years, one third of Britain's power-generating capacity needs to be replaced with cleaner fuels. But last night the Conservatives said that Labour had refused to face up to the problem.

The admission that Britain will face power-cuts is contained in a document that accompanied the Government's Low Carbon Transition Plan, which was launched in July.

Ed Miliband, the Energy and Climate Change Secretary, outlined the plan amid much fanfare.

Under the plan, 40 per cent of the UK's electricity will need to come from low-carbon energy sources including clean coal, nuclear and renewables.

Accompanying the report is an appendix, only published online, which warns of power shortages. It details supplies and expected demand between now and 2030.

It highlights the first short-fall in 2017. The "energy unserved" level reaches 3000 megawatt hours per year.

That is the equivalent of the whole of the Nottingham area being without electricity for a day.

By 2025 the situation worsens with the shortfall hitting 7000 megawatt hours per year. That is the equivalent to an hour-long power cut for half of Britain.

Greg Clark, the shadow climate and energy change secretary, said: "Britain faces blackouts because the Government has put its head in the sand about Britain's energy policy for a decade. Over the next 10 years we need to replace one third of our generating capacity but Labour has left it perilously late, and has been forced to admit they expect power cuts for the first time since the 1970s.

"The next government has an urgent task to accelerate the deployment of a new generating capacity, and to take steps to ensure that as a matter of national security there is enough capacity to provide a robust margin of safety."

Mr Clark also pointed out that the scale of the blackouts could in fact be three times worse than the Government predictions. He said some of the modelling used was "optimistic" as it assumes little or no change in electricity demand up until to 2020.

It also assumes a rapid increase in wind farm capacity. There is also the assumption that existing nuclear power stations will be granted extensions to their "lifetimes".

The last time Britain experienced regular power cuts because of shortages of supply was in the early 1970s, when a miners' strike caused coal restrictions. The country was forced to do everyday tasks by candlelight and a three-day week was imposed on all but essential services to try and conserve electricity.

The looming problem in Britain is caused by the scheduled closure by 2015 of nine oil and coal-fired power plants. They are the victim of an EU directive designed to cut pollution.

In addition, four existing nuclear power plants are set to be shut, adding to the need for new sources of energy.

Labour failed for several years to commit to a new generation of nuclear power stations. Several reviews and rows with the green lobby delayed any definitive statement on the issue.

The Government has now given the go-ahead for a new generation of nuclear power, but it remains up to private energy firms to build the plants. Ministers have been urged by business and the power companies to provide more incentives to make building new power stations more attractive.

As yet no new nuclear power stations have been put forward for approval by the Government.

The privatised power stations have been slow to commit to building new capacity because of Government intransigence and their own misgivings over whether profits can be made.

If plans are approved for new nuclear power stations then they will trigger public consultations and possible inquiries, further delaying the day when new electricity sources can be switched on.

It means that there will not be any new nuclear power stations before 2018. Any drive for renewables, in particular wind farms, is unlikely to meet the gap left.

Under Mr Miliband's new plan it is predicted that by 2020 there will be around 30 per cent of electricity coming from renewables, 10 per cent from Carbon Capture and Storage - even though the technology is still not certain - and only about 8 per cent from nuclear, which is about half of the current level.

There will be huge reliance in the short term on gas, with up to 50 per cent of electricity coming from gas fired power stations.

Despite the belated commitment for new nuclear power stations, Mr Miliband has expressly ruled out giving any financial help to companies contemplating the move. The CBI had hoped that the Energy and Climate Secretary would help by putting a floor of the carbon price and therefore acting as an incentive.

Only a few existing nuclear sites like Sizewell B in Suffolk will still be generating electricity in 2020.

A spokesman for the Department of Energy and Climate Change said: "We are moving in the right direction towards low carbon energy but we are

in transition, we can't just click our fingers and expect to end carbon emissions overnight. In the near turn there will be a need for the continued use of fossil fuels.

"We're determined the imports of those fuels should be diverse and the emissions from them capped by the emission trading scheme. We will need to import more gas in the short term - that's why diversifying our import options for gas is important - something else that is happening already."