Wind farm firms have been accused of building huge, ineffective turbines to exploit a lucrative loophole funded by the taxpayer.

And although the Government knows about the scam, it has not acted to stamp it out.

The Government pays different rates for wind energy depending on how much power is produced by turbines. In an effort to encourage small businesses and individuals to get involved in the industry, David Cameron’s coalition agreed to buy electricity produced by low-powered machines at around double the rate of towering turbines. This means businesses like farms can afford to run a small turbine, which does not produce huge amounts of electricity.

But some operators are exploiting a legal loophole by building huge turbines and then slowing them down so their output is within the same category as a much smaller machine.

Critics claim it can be highly lucrative because owners receive the higher Feed-In Tariff (FIT) rate but also have a giant turbine which will consistently out-perform smaller machines.

But the practice, known as de-rating, means that some of the huge turbines scarring the landscape have been deliberately modified to be ineffective.

The Sunday Post has learned that although the Westminster Government is aware of specific de-rating cases, it has not moved to close down the loophole.

Scottish Conservative MEP Struan Stevenson last night blasted: “The whole thing is getting exposed as one of the biggest scandals since the collapse of the banks and de-rating is simply another spoke in the wheel.”

Labour MP Sir Tony Cunningham, who represents Workington in Cumbria, recently quizzed the Westminster Government to find out what action it was taking. In response to his parliamentary question Energy Minister Michael Fallon revealed he was aware that eight of 110 turbines installed at the higher 100kw to 500kw FIT rate up to September 2013 had been de-rated.

He also revealed talks with industry body RenewableUK had not identified a “workable technical solution”.

Linda Holt, of campaign group Scotland Against Spin, said: “Consumers are being ripped off. They are being forced to pay more for the turbines and people have suffered greater visual impacts than they need to.”

Regulator Ofgem, which licenses the FIT scheme, said it does not keep a list of how many turbines on the FIT scheme are de-rated. But when it receives applications for a modified turbine it makes stringent checks to ensure the turbine has been permanently downgraded. It also confirmed it has not yet rejected any applications for the coveted 100kw to 500kw FIT category.

RenewableUK’s deputy chief executive Maf Smith said: “The wind industry adheres strictly to the guidelines drawn up by the Department of Energy and Climate Change and the independent regulator Ofgem.

“When issues have arisen, we have drawn them to the attention of Government and regulators, recommending improvements to ensure that the system is robust. The reasons for de-rating are complex. In some instances, the grid is unable to cope with a turbine operating at full power, as grid connections are limited in that area.”

Department of Energy and Climate Change spokesperson said: “This is not a widespread problem and there is little evidence that de-rating is used as a means of accessing preferential tariffs.”

Wind farms were “secretly” paid nearly £20m to shut down before spells of stormy weather, an investigation revealed.

Companies qualify for “constraint payments” when they have to temporarily close down their turbines because bad weather would mean they produce so much power the National Grid would be unable to cope. The cash is paid to the companies through householders’ domestic bills.

Dr Lee Moroney, of the Renewable Energy Foundation, uncovered a little known system called “forward trades” in which the Grid decides a sum that will be paid for a period of heavy weather, which is agreed before the bad weather even arrives.

It revealed £18.6m in forward trades were paid in 2011/12 in addition to £15.5m in traditional constraint payments. The payments covered all forms of power generation in England and Scotland but it is understood the majority applied to wind farms.

The Feed-In Tariff is a Government scheme in which fixed-rate payments are made for every kilowatt hour generated by a turbine through a “generation tariff”.

Turbines with a capacity of between 100kw and 500kw which come online before March 31 will earn 18.04p per kilowatt hour of electricity and those which generate 500kw to 1.5m kw earn 9.79p p/kwh. But the tariffs will be reduced for turbines coming online after April 1 with 14.82p p/kwh for turbines which produce between 100kw to 500kw and 8.04p p/kwh for 500kw to 1.5m kw machines.

Turbine owners can also use the electricity to power their businesses thus saving thousands of pounds in energy bills. They also see a second benefit from an “export tariff” in which excess energy not used by the turbine owner can be sold to the National Grid for 4.64p p/kwh.

Critics reacted with fury when it was revealed millions of trees had been felled to make way for wind farms.

According to figures released in 2011, 10,000 hectares of woodland had been felled over the past decade to allow giant turbines to be built. It meant an area covering almost twice the size of Dundee could have been felled to fuel Scotland’s “renewables revolution”.

Critics hit out at the destruction of the forests which naturally soak up C02 emissions. John Mayhew, of the Association for the Protection of Rural Scotland warned wind farms were the biggest threat to Scotland’s rural habitats and landscapes.

Last week The Sunday Post revealed tycoon Donald Trump was facing a fresh battle over wind turbines at his new golf course in Ireland.

He recently withdrew plans to build a second golf course in Aberdeenshire, after losing a legal battle to stop construction of 11 turbines off the coast. He then revealed he had invested £12.4m in the Doonbeg Golf Club in County Clare instead.

But a planning application has been lodged for nine giant turbines to be built three miles inland from the course.

Environmental campaigners say they will be contacting Mr Trump to ask for his support in opposing the plans.

In November The Sunday Post revealed a Scots dog owner had won a battle to have two wind turbines removed after claiming her pet suffered seizures.

But 66-year-old Irene Cardle’s victory was tinged with sadness because her beloved dog Shadow died just days after the 19-yard machines came down.

Irene claimed Shadow’s health seriously deteriorated after nearby Blacklaw Primary School, in East Kilbride, built two turbines close to her home. The retired book-keeper revealed the turbines had made their lives a misery and she was forced to leave the house for hours at a time to escape the constant flicker and whine.

South Lanarkshire Council said it removed the turbines because they were not “cost-effective”.