Nato has begun an investigation into British findings that wind farms make overflying planes invisible to radar as military chiefs fear a security threat from the rapid spread of the turbines.

The US has been attending tests by Britain’s Air Warfare Centre after it made the surprise discovery that the energy plants create blind spots in air defences.

Renewable energy campaigners have been stung by a spate of last-minute objections from the Ministry of Defence to proposed new wind farms in northeast England and the Scottish Borders.

Nato’s alarm about this potential Achilles’ heel against airborne terrorists or invaders is disclosed in evidence, seen by The Times, for a planning inquiry.

The MoD is now objecting routinely to all wind farms within line of sight of radar stations, irrespective of distance. There is currently no known technical solution.

Evidence was given by Squadron Leader Chris Breedon, opposing a 48-turbine wind farm at Fallago Rig in the Lammermuir Hills in Scotland. “As a result of MoD trials proving that wind turbines adversely influence the performance of military and civilian radar systems operating within radar line of sight, Nato has become concerned about the rapid increase in the number of wind turbine farm projects under planning or in development in a number of Nato countries,” he said.

Britain is leading an investigation by the Nato Research & Technology Organisation into the impact of wind turbines on radar. The first meeting of a newly created technical group, involving the US, France, Italy, Norway, Belgium, the Netherlands and Greece, took place in London in June. The experts will review all scientific evidence from trials, consult the wind farm industry and civil aviation authorities, decide what new trials are needed and recommend policy changes.

A cloud will hang over the wind farm industry for years as the alliance’s report is not due to be presented to Nato’s sensors and electronic technology panel until 2010. A review will take place the following year.

Britain discovered the blind spots during tests over a Welsh wind farm in 2004. Pentagon experts were invited to observe subsequent trials.

President Bush’s Administration was so anxious initially that it introduced an immediate moratorium on all wind farms in line of sight of its own military radars. Since then the stance has been softened and each new US wind farm is now considered on a case-by-case basis.

There is still no sign of a solution to the British impasse caused by the MoD’s objections to wind farms in line of sight of its radar stations. Although Britain refuses to say how far the line of sight extends, a Pentagon report suggests a 60-mile radius.

The problem is urgent because the stations tend to be on the east coast and the North Sea has been earmarked for a major expansion of offshore wind farms.

Since 9/11, radar policy has been dominated by fears of an airborne terrorist attack launched inside British air space, rather than an invasion from overseas. Air Chief Marshal Sir Jock Stirrup, Chief of the Defence Staff, has given a firm direction that radar surveillance capability must not be degraded.

Despite the MoD’s stance, John Hutton, the Energy Secretary, went ahead with an announcement in December of 7,000 new turbines by 2020.

In a key climate change speech last autumn, the Prime Minister said that he had asked Des Browne, the Defence Secretary, and Mr Hutton “to step up their efforts . . . and test technical solutions to the potential difficulties that wind farms pose to air traffic and defence radar”.

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