A cost-benefit analysis claiming that harm to the environment caused by Birmingham International Airport's runway extension will be outweighed by economic benefits to the West Midlands has been condemned as seriously flawed.

Friends of the Earth and Birmingham Airport Anti-Noise Group BANG say the study by consultants York Aviation fails to take properly into account issues relating to climate change.

The two groups are stepping up their campaign by urging people living close to the airport to express their concerns to Solihull Council before February 22, the cut-off date for comments on the runway extension planning application.

A demonstration held in Solihull town centre last Saturday resulted in almost 200 shoppers agreeing to write letters of complaint to the council.

Activists fighting the runway plans dressed as pilots and air stewardesses and handed out "Flyagra" pills.

The pills, in reality foam earplugs, were meant to highlight what campaigners claim will be additional noise and pollution if the 400-metre runway extension is approved.

The longer runway, which could be in place by 2012, is regarded as essential by West Midlands councils and business organisations in order for BIA to offer non-stop flights to China, India and the west coast of America.

Failure to deliver a longer runway could cost the local economy millions of pounds in lost business to the North-west and South-east of England, they claim.

Airport expansion is now at the top of West Midlands councils' transportation wish list, following Government approval for the refurbishment of Birmingham New Street Station last week.

But BANG and Friends of the Earth question the validity of York Aviation's analysis.

Secretary of BANG, James Botham, said: "We know from the official environmental impact assessment submitted by the airport company as part of its planning application that noise pollution and carbon dioxide emissions from the airport will increase over the next 25 years if the runway extension goes ahead as planned.

"Airport bosses and the local business lobby are happy to trade off the growing environmental impact against what they perceive to be the greater economic good.

"Consultants for BIA have concluded that the potential benefits of the runway extension outweigh the costs, but this analysis is flawed, mainly because the climate-change costs of the runway extension have been seriously underestimated.

"In reality, it is more likely that the economic and environmental costs of the runway extension outweigh the benefits."

Mr Botham said York Aviation's cost-benefit analysis included an estimate of the future economic cost of the extra carbon dioxide emissions from planes using the extended runway, when in fact the total global warming impact of aviation is two to four times greater than the impact of aircraft carbon dioxide emissions alone.

A BIA spokesman said: "Their approach is very amusing but global warming is a serious concern, so much so that we'd ask why Friends of the Earth and BANG don't concentrate more on the 98 per cent of Co2 emissions that are not aviation related."

Solihull Council has until the end of April to decide on the runway planning application.