It ruled that Renewable Energy Systems (RES) had used misleading figures about its potential impact on global warming.

A leaflet about plans to erect wind turbines on farmland near North Tawton claimed they would reduce CO2 emissions by over 1.3m tonnes in their lifetime.

The ASA ruled this figure should not have been quoted without qualification.

Campaigners are now calling on all renewable energy firms to revise their claims about the value of onshore wind power.

Leaflet withdrawn

RES told the ASA it calculated the figure by comparing the wind farm's predicted CO2 emissions over its 25-year lifespan with those of a typical coal-burning power station.

Coal produces significantly more CO2 emissions than gas or nuclear power - a coal plant gives off 860g of CO2 pollution per kilowatt-hour of electricity generated, compared to the UK annual average of 430g.

The wind industry as a whole must now revise its claims, claims which have seriously distorted debate about the value of onshore wind power Dr John Constable

RES said electricity from wind turbines would replace the output of coal-fired power stations because they are the most flexible part of the national grid.

The ASA ruled this was a reasonable basis for calculating the reduction in emissions at the present, but not over a period of 25 years because of uncertainties about which fuels the UK would use in the future.

Dr John Constable, policy and research director of the Renewable Energy Foundation, said: "It's good to see that the ASA has revisited this issue and brought its ruling into line with common sense engineering principles.

"The wind industry as a whole must now revise its claims, claims which have seriously distorted debate about the value of onshore wind power."

The ASA initially rejected all objections about the leaflet, distributed in November last year, but today overturned its ruling on one of the complaints.