Although it objected to a crematorium near Melrose in the Borders because it would spoil a national scenic area, it raised no objection to a development of 1500 houses in the Cairngorms national scenic area, near Aviemore.

This is “extraordinarily inconsistent”, say environmental groups, who are fearful that SNH is weakening the protection it gives to important landscapes, sites of natural beauty and wildlife areas by allowing damaging developments to go ahead.

The Scottish Campaign for National Parks has written to the chairman of SNH, Andrew Thin, criticising the agency’s decisions.

“Things seems to have changed in recent times,” said the campaign’s chairman, Robert Maund. “We no longer feel able to rely on SNH to provide consistent advice and, where necessary, firm objections on development proposals which can do serious damage to the landscape and sensitive habitats.”

Maund pointed out that SNH had failed to block the An Camas Mor housing scheme in the Cairngorms, though there was “no credible reason” why the development should be allowed to mar a scenic area. This contrasted with the agency’s opposition to the crematorium near Melrose.

“It does seem apparent that the responses from SNH on the two cases lack consistency,” argued Maund. There were “numerous other examples” of SNH coming to contradictory decisions, he claimed.

In the Clyde Muirshiel Regional Park, SNH has objected to some wind farms, but not others. “In one notorious case the objection was withdrawn shortly before the inquiry commenced, totally undermining the park authority, the local planning authority and the community,” Maund said. “It is absolutely vital in this pressured world, when development seems to be treated as a ‘good thing’, that there is a guardian for the environment which will speak out without fear or favour when Scotland’s landscapes and habitats are being threatened.”

Maund’s accusations have been backed by Badenoch and Strathspey Conservation Group. The group’s convenor, Dr Gus Jones, said such “inconsistency makes a mockery” of how Scotland protects its areas of national beauty, and also “fails the public interest”.

He said in 1994, SNH said a much smaller scheme at An Camas Mor would dramatically impact on the landscape. The agency had also objected to the £1.3billion plan for a luxury holiday resort in a scenic area on the shores of Loch Rannoch in Perthshire.

“It is extraordinarily inconsistent of SNH,” alleged Jones. He added that he believed SNH was “abdicating its responsibility” for protecting both Scotland’s world-famous scenery and the nation’s “protected species.”

In another case, SNH has been criticised for failing to take account of the risk to a rare wild plant that was posed by a proposed hydro-electric scheme on the Birks of Aberfeldy, in Perthshire. According to Rhiannon Crichton, from the Royal Botanic Garden in Edinburgh, it was the second most important site in the country for the small cow wheat.

SNH, however, strongly defended its decisions.

“We must combine consistency of overall approach with sensitivity to local circumstances,” said an SNH spokesman. “In commenting on individual development proposals, we obviously have to assess them on the basis of their specific impacts. These can vary widely even between similar types of development quite close to each other.”

In the case of the Cairngorms, he pointed out, it was up to the national park authority to comment on the landscape impact of developments. “That is what happened with regard to the An Camas Mor development,” said the spokesman.

“In the Melrose crematorium case, SNH was responsible for assessing the effect on the national scenic area and we duly provided our advice.”