In a damning report, published to coincide with a meeting in Edinburgh today between the developers and MSPs, the Royal Society for the Protection of Birds Scotland claims that the £411 million development, involving the erection of 234 wind turbines across 43km, will have a "catastrophic" effect on more than 6,000 hectares of protected peatland other vital habitats.
RSPB Scotland has warned that, should the development get the go-ahead, it will set a "very dangerous precedent" for all internationally important sites in Scotland threatened by similar developments.
The charity has also produced a series of maps to illustrate the scale of the development, superimposing the gigantic wind farm on key population centres.
The maps show that the development would stretch north from Edinburgh to beyond Methil and west to Dunfermline; from Glasgow’s Central Station to East Kilbride in the south and north to Falkirk; and from Epsom Downs, south of London, to several miles north of the Thames flood barrier and east to Hampton Court.
This year Western Isles Council decided by 19 votes to eight to support the proposed development by Lewis Wind Power, a consortium led by British Energy and Amec.
The Executive, which will have the final say, has received more than 4,200 representations on the proposal, with just nine in favour. Representatives of British Energy and Amec are due to meet MSPs in the Scottish Parliament today to brief them.
But the RSPB is calling on the Executive to reject the scheme because of its "disastrous" implications for rare birds and wildlife.
James Reynolds, a spokesman for RSPB Scotland, said: "British Energy and Amec’s application to build the 702-megawatt wind farm will have a staggeringly damaging impact on an internationally important and sensitive location, protected by multiple nature conservation designations.
"For certain species, including golden plover and dunlin, the peatland habitat where the project will be sited is the best in Europe, and hosts a significant proportion of their total British or world populations.
The RSPB believes that, although the environmental survey work undertaken is extensive, it is fundamentally flawed."
David Hodkinson, a director of Lewis Wind Power, refuted the RSPB claims. He said: "Our environmental work and proposed design have moved forward considerably since our planning application was submitted in the autumn of 2004. We are now confident that we would be able to minimise the effects of the wind farm on birds through a combination of measures."
The company intends to present its updated proposals to the Scottish Executive and its advisers in the New Year.