"Our understanding is that the Ministry of Defence will make formal objections to five sites, each comprising 30 turbines," Hill said.

Commenting on a report in the New Scientist magazine, she added that the wind farms would be key if the government wanted to reach its target of obtaining 10 percent of electricity from renewable sources by 2010.

"If they were all built, the 18 sites would provide more than one percent of UK electricity supply," she said.

"The government's own legal requirement is that 10 percent of electricity is from renewable energy by 2010. It is widely expected that wind power on and offshore will provide half of that."

A defence ministry spokesman said he could not confirm that five sites would be blocked.

New Scientist said four of the threatened wind farms were in the Irish Sea off the coast of Lancashire and the fifth was in the North Sea off Norfolk, eastern England.

"I can confirm that one (proposed wind farm) is definitely blocked on radar grounds and that is the one off Cromer in Norfolk," a spokesman said. "The rest I cannot immediately confirm."

The BWEA said that using radar-reflective paint or simply moving the giant turbines could solve the problem.