A controversial golf course planned for northwest Scotland will be damaged by coastal erosion caused by climate pollution, according to expert advice from the Scottish Government’s wildlife agency.

Documents released under freedom of information law reveal that Scottish Natural Heritage (SNH) officials believe that the proposed Coul Links course in Sutherland is not sustainable because greens will be submerged by storms, sea and sand.

The private advice from officials is significantly more forthright than SNH’s caveated formal letter of objection. This has prompted a leading environment group to accuse SNH of “sugar-coating” its objection.

US property developers have applied for permission to build a golf course on the northeast coast near Dornoch. But experts say it will damage a precious and heavily protected natural sand dune system.

SNH has released more than 500 pages of internal reports, memos and emails on Coul Links over recent years. The agency’s north highland area officer, Alec Macdonald, repeatedly warned that four tees – 15, 16, 17 and 18 – were particularly vulnerable to coast erosion.

“Elements of the development cannot be adequately safeguarded over the lifetime of the development without the construction of coastal defences," he said in a memo on 27 October 2017.

"The layout of the course fails to take into account the developer’s own advice, nor the clear evidence of increasing anticipated erosion during the lifetime of the development.”

Macdonald said the development breached Highland Council’s policies on coastal development and its duties on climate change.

SNH’s insect expert, Dr Athayde Tonhasca, cautioned that Coul Links could make the extremely rare Fonseca’s seed-fly extinct. “The species may eventually be wiped out from the whole coast as a consequence of the golf course development,” he said.

According to coastal ecologist Professor Stewart Angus could not meet SNH’s stated policies. "The tees, greens and fairways as envisaged in the draft layout supplied to SNH will result in an unacceptable level of damage to nationally important and designated dune habitat,” he said.

SNH’s official letter of objection in November 2017 opened by highlighting “the potentially large economic benefits” and praising the developers’ “commitment and creative thinking”. It then said that Coul Links is “contrary to the Scottish Planning Policy’s requirements.”

The Royal Society for the Protection of Birds in Scotland said it was pleased that internal advice was “so unequivocally opposed” to a “nature-wrecking” project, but its head of planning Aedán Smith said: “It is extremely puzzling that SNH felt the need to sugar-coat their otherwise robust objection by talking up some possible economic benefits.

“This mixed messaging will only have given the developer false hope, when there is almost none, and encouraged them to keep trying to build on this completely inappropriate site.”

The chair of the local Not Coul group, Dr Tom Dargie, described SNH’s concerns about climate change as “spot on”. The risk of coastal erosion was “high and immediate,” he said.

“The back tees for holes 16 and 18 are already only three and seven metres respectively from an eroding dune cliff, with the fairways and greens of holes 15 and 17 likely to be threatened by dune retreat inland within a decade or earlier.”

The developers insisted that SNH’s advice had been taken into account and that “numerous changes” had been made to the course layout over the past two years. “With respect to the Fonseca’s seed-fly, we conducted the first ever site study and altered the layout of our course away from known locations,” said a spokesman for the developers, Todd Warnock and Mike Keiser.

“Further, we have agreed to fund an independent academic study of the species in a wide area ranging from Brora to southern Dornoch. We firmly believe our environmental strategy will result in an improvement in the Site of Scientific Interest as well as material economic benefit to an area in dire need of investment.”

SNH confirmed it had offered Highland Council advice. “Our response draws on the knowledge of many experts,’ said the agency operations director, Nick Halfhide.

“We recognise the potential significance of the proposal to the local economy, but we aren’t able to support in its current form, as it would remove 16 hectares of nationally important sand dune habitat.”