An area of unspoilt Scotland the size of Sutherland has been lost to development over four years, according to new figures.

Conservationists who have raised fears over the visual impact on Scotland's scenery say the proliferation of wind farms is largely to blame.

The study carried out by ­Scottish Natural Heritage reveals a dramatic decline in the nation's countryside, with building carried out on nearly 2000 square miles of unspoilt Scotland over one four-year period alone.

Conservation groups fighting to ensure Scottish Government planning regulations provide better protect the nation's precious landscape fear it has already been "trashed".

The developed area of Scotland rose from 65% (19,890 square miles) in 2008 to 71.4% (21,732 square miles) in 2012.

The research reveals the key reason for the dramatic change in recent years is wind turbines.

An analysis of that four-year change shows the proportion of visual impact caused by turbines has more than doubled, from 19.9% in 2008 to 41.7% in 2012.

The SNH research also shows the proportion of Scottish land that could be counted as being "without visual influence of built development" has dropped by 31% over 10 years from 12,476 square miles in 2002 to 8,824 square miles in 2012.

Twelve years ago it was ­estimated 41% of Scotland was unspoilt by development. In 2012 it was 29%.

Alex Salmond has set a target of generating the equivalent of all of Scotland's electricity from renewable sources by 2020 but critics say no calculations have been made for the numbers of onshore wind farms required to meet this.

The Mountaineering Council of Scotland says it has become increasingly concerned about the intrusion of wind farms on to mountains and moors with no sign of a curb to their expansion.

Dave Gordon, the body's ­director of landscape and access, said: "We recognise that onshore wind has a role to play in a balanced energy mix, but the current generously incentivised mania for turbines is producing an acutely unbalanced energy mix. We are behaving in a totally unsustainable manner in the name of sustainability.

"Developers won't stop coming until the money tap is turned off. They don't care about Scotland, just their profits."

The National Trust for Scotland said: "Our research found that development affecting Scotland's landscapes is a concern for people, who feel these places should be protected. The impact of adverse ­developments - especially pylons and wind turbines - on lochs and mountains, coastal landscapes and historic settings is a worry."

Calum Brown of the Scottish Wild Land Group added: "I think the wild land is being destroyed and industrialised by wind farms which have little impact on climate change. Wild land is a precious resource and must be protected, but still wind farms keep popping up."

John Mayhew, director of the Association for the Protection of Rural Scotland, said: "A lot of the damage has been done. But the Scottish Government's wish to expand onshore wind farms means there is more to come."

Ministers who are currently considering a revamp to planning rules in Scotland have promised to protect "wild land". The National Planning ­Framework and review of Scottish planning policy being considered could see wind farms being outlawed in the country's national parks and designated scenic areas.

But conservation groups have told the Scottish Government the measures are too lax.

A Scottish Government spokesman said it wanted to see the "right developments in the right places". He added: "Scottish planning policy is clear that the design and location of any wind farm should reflect the scale and character of the landscape and should be considered environmentally acceptable."

The finalised Scottish planning policy will be published in June.