The study which examined the performance of wind farms in the UK and Denmark says the economic life of onshore turbines is between 10 and 15 years, not the 20 to 25 projected by the wind industry itself and used for Government energy projections.

It says that by 10 years of age, the contribution of an average UK wind farm to meeting electricity demand has dropped by a third.

This means it is rarely economic to operate them for more than 12 to 15 years, after which they must be re-powered with new machines, it is claimed.

The study commissioned and published by anti-wind farm charity the Renewable Energy Foundation (REF) is said to have important implications for UK wind policy.

The findings suggest that the current subsidy regime is "extremely generous" if investment in new wind farms is still profitable despite the decline in performance due to age and wear and tear.

Yesterday Dr John Constable, director of REF, said: "This study confirms suspicions that decades of generous subsidies to the wind industry have failed to encourage the innovation needed to make the sector competitive.

"Bluntly, wind turbines, onshore and offshore, still cost too much and wear out far too quickly to offer the developing world a realistic alternative to coal."

The study, which was carried out by leading energy and environmental economist, Prof Gordon Hughes of Edinburgh University, applied statistical analysis to years of actual performance data from wind farms in both the UK and Denmark.

The results show that, after allowing for variations in wind speed and site characteristics, the average load factor of wind farms declines substantially as they get older, probably due to wear and tear.

It says policymakers expecting wind farms built before 2010 to be contributing towards CO2 targets in 2020 or later must allow for the likelihood that the total investment required to do so will be much larger than previous forecasts have suggested.

Prof Hughes said the study has important implications for UK policy on wind generation. "Some investors will be aware of the decline in performance, but nevertheless continue to invest, suggesting that the subsidies are so generous as to compensate for the fall in output," he said. "Therefore there is probably room for further subsidy reductions to cut costs to the consumer."

He also says the structure of contracts offered to wind generators should be changed to reflect the shorter economic life of turbines, and claims meeting targets for wind generation will need a higher level of wind capacity and capital investment than previously thought.

Last night anti-wind farm campaigner Andrew Joicey, a farmer at Cornhill, Northumberland, said: "This is quite a significant study and will make would-be investors sit up and think, now that they understand the physical constraints on wind turbines.

"It reflects anecdotal evidence that people driving past wind farms, that have been there for a few years, see turbines which aren't turning. That's because they are starting to break down.

"If they only last 10 to 15 years that puts a very different picture on the whole question. It is the subsidy system that makes onshore wind so spectacularly profitable, and this study shows there is a strong case for reducing that subsidy."

Don Brownlow, who lives near Berwick and runs the Windbyte website, said: "All this study does is reinforce what we have known for a long time; that wind industry claims don't add up. There are examples of wind farms in the North East being re-powered after eight or nine years. With subsidies, investors are getting payback within six or seven years, so even if the turbines fail at 12 years they are still making rather large profits."

But Dr Gordon Edge, director of policy at RenewableUK, the trade body for the wind industry, said: "Better turbines are being developed all the time, so it's absurd to focus purely on the past, as this report does, and pretend that's the way things are going to be in the future.

"If what REF is claiming were true, then the industry simply wouldn't be able to raise money. The fact that investors have remained confident in the wind energy sector demonstrates their confidence in the technology.

"All-importantly, wind farm developers only earn money for the clean electricity they actually generate, so it's very much in their interests to make sure that their turbines are maintained throughout the 25-year lifespan of the wind farm to an optimum level, which includes upgrading as the technology improves."