Critics say the Government's Renewable Energy Strategy fundamentally underestimates how many wind turbines will be needed to meet its ambitious targets for a reduction in the use of fossil fuels.

Ed Miliband, the Energy Secretary, last week announced that planning rules would be relaxed to make it easier for an extra 3,500 onshore turbines to be built as part of a £100 billion plan to generate more energy from wind power by 2020.

Ministers want to increase the amount of power coming from renewable energy from two per cent to 15 per cent by that date, in line with European Union targets.

In order to achieve this the Government says it needs to increase the number of onshore turbines from the current 2,500 to 6,000 and build 4,000 at sea, bringing the total number of turbines to 10,000.

Mr Miliband said: "We want to do more and go further and faster. We need to change the default position so that people will come to understand the dangers of climate change to our beautiful countryside."

But a number of scientists and environmental energy experts have dismissed the Government's ambitions as "wishful thinking".

An analysis of the key 15 per cent target, carried out by the Renewable Energy Foundation (REF) think tank for The Sunday Telegraph, reveals it is based on the assumption that Britain's total energy consumption will fall in coming years.

However energy experts point out that population growth and future economic recovery is likely to drive up the demand for energy, even if Mr Miliband's proposals to improve energy efficiency in homes and workplaces are introduced.

They say that means even more wind farmsthan the Government has admitted to would be required to meet the 15 per cent target.

Dr John Constable, director of policy and research at the REF, said: "By assuming that UK energy consumption will actually fall by 2020, in spite of rising population and economic growth, the Government has lost any claim to credibility.

"Despite improvements in energy efficiency across OECD countries, energy consumption continues to increase."

The consultation document on the Renewable Energy Strategy published last year was based on the UK's total energy consumption remaining at current levels of 1,750 Terawatt-Hours of energy a year, up to 2020 - an amount it would take Sizewell B nuclear power station 200 years to produce.

That would have required around 260 TWhs of energy to be produced from renewable sources for the 15 per cent target to be met.

But the final strategy document goes one step further and predicts a fall in energy demand to 1,600 TWh, requiring some 240 TWh of energy to be produced from renewables.

The REF however has predicted that if energy consumption grows, as it says is more likely, some 300 TWhs will have to be produced from renewables such as wind and wave power to meet the target, a 25 per cent increase on the Government's estimate.

Dr Constable added: "The Government's Renewable Energy Strategy is wildly and irresponsibly optimistic about the scale of the targets and so underestimates the risks, the difficulties and the costs facing the UK."

Critics of the Renewable Energy Strategy say that even building thousands of 300 ft high wind turbines across Britain will only have a minor impact on cutting the country's use of fossil fuels while still blighting much of the countryside.

REF claims that the Government's strategy overestimates the benefits of an increased reliance on renewable energy sources.

The strategy predicts it will save 750 million tonnes of CO2 between now and 2030, at a rate of 37.5 million tonnes CO2 per year. But that represents just seven per cent of the UK's annual CO2 emissions and only 0.1 per cent of world emissions.

"This is not a good bargain and reinforces the point we have often made that renewables are poor emissions reducers, whatever other virtues they might have," said Dr Constable.

Professor Ian Fells, an energy expert and Fellow of the Royal Academy of Engineering, said the Government's plans were "hugely expensive" and "wildly optimistic" as they required 10 off shore wind turbines to be erected every day with good weather over the next decade.

He also claimed the strategy failed to address the impending shortage of electricity generating capacity.

He said: "Up to one third will disappear over the next decade as ageing nuclear power stations are decommissioned and 40-year-old coal-fired stations face closure because they fail to meet EU emissions standards.

"According to the paper, new build nuclear stations cannot be ready before 2018 and, in some cases, 2025, yet we face the possibility of rolling blackouts as soon as 2013.

"Fixing this gap will be difficult and probably mean more gas fired stations using gas imported from Russia."

Under the Government's plans wave power and tidal energy will also be harnessed to increase Britain's renewable energy production.

The Renewable Energy Strategy estimates that household energy bills could increase by as much as £250 a year in order to pay for measures to reduce Britain's carbon footprint, though Mr Miliband said the actual rise would be closer to £77 as a result of more energy efficient households using less gas and electricity.

Supporters of wind power have backed Mr Miliband's plans. Maria McCaffery, chief executive of the British Wind Energy Association (BWEA). said: "We welcome the government's commitment to delivering on the 2020 targets.

"They have rightly ignored the siren calls to abandon wind as the driving force for reaching the targets. This will help convince investors that the country is serious about fighting climate change and developing domestic, renewable sources of energy."