A leaked plan by the hunting lobby to remove pine martens from ­Scottish ­woodlands has sparked fierce opposition from wildlife groups which suspect landowners of planning a cull to protect their lucrative grouse stocks.

The Game & Wildlife Conservation Trust (GWCT), a charity funded by landowners, farmers and sporting interests, wants to launch a trial next year to trap and relocate pine martens, which are protected under law as an endangered species.

But the plan has been condemned as "deeply flawed" by experts who fear it could open the door to the widespread killing of pine martens. It has also been rejected by the Royal Society for the Protection of Birds (RSPB), which owns two of the woodlands where the trial had been proposed.

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A six-page internal document drawn up by GWCT scientists discloses a plan to get rid of 120 pine martens over six years from four Strathspey forests. It promises to release any females caught in traps "on welfare grounds" if they are suckling young.

Three of the forests - Rothiemurchus, Glenmore and Inshriach are owned by the Forestry Commission and one - Kinveachy - by the 13th Earl of Seafield. GWCT had to abandon plans to use two other woodlands - Abernethy and Craigmore - because of opposition from their owner, the RSPB.

"Whilst humane killing of captured martens may be the ­easiest option, we intend to make martens available for wider marten conservation programs," the leaked document says. "These may include reintroduction programmes to England and Wales."

The purpose of the trial is to test whether martens are harming the prospects of another endangered species, capercaillie, by eating their eggs and chicks. These birds have declined from 20,000 in 1970 to about 1200, many of which share ­Strathspey woodlands with martens.

But conservationists suspect ­ulterior motives. They point out that landowners and gamekeepers have been increasingly pushing for the right to cull pine martens in order to protect red grouse so they can be shot for sport.

The Mammal ­Society, which promotes science- based conservation, has written to the Government's wildlife agency, Scottish Natural Heritage (SNH), expressing "deep concern" about the proposed trial. Evidence in support of it was poor, the society said, and there were no organisations willing to take any relocated martens.

"If this trial were to go ahead it is very likely to generate immense negative publicity both for SNH and the Scottish Government, not just within the UK but internationally," the society concluded. "This will be particularly vociferous if the fate of the removed pine martens is to be culled. It will also greatly damage the reputation of Scotland as a destination for wildlife tourism."

One of letter's authors was the chairman of the Mammal Society and a leading pine marten specialist, Dr Johnny Birks. "Instead of pursuing a flawed scheme to remove it, we should be celebrating the marten's return to Speyside," he said.

"Unlike our Victorian ancestors, thankfully we now understand that predation is a natural component of healthy ecosystems. Pine martens have coexisted in a stable predator-prey balance with capercaillie in forests across northern Europe for thousands of years."

Pine marten predation is a peripheral factor in the ­capercaillie's decline, Birks argued. ­"Predators frequently influence the breeding success of their prey, but this is a natural process and should not be used to justify the removal of one protected species to protect another."

Duncan Orr-Ewing, head of species and land management at RSPB Scotland, dismissed the proposed trial as inappropriate and unnecessary. "We have not agreed to this happening on any RSPB Scotland nature reserves," he said. "We are proud that martens and capercaillie are found in our forests."

In the leaked document, GWCT claims the backing of the Forestry Commission and SNH for its trial and says the Cairngorms National Park Authority "may" be a partner. But all these agencies told the Sunday Herald they had yet to decide whether or not to go ahead with removing martens.

Hamish Trench, the park authority's conservation director, said there were "significant questions" about a trial removal of pine marten. The Forestry Commission insisted the idea was still being developed, and SNH said it was too early to take a view "on the merits of the work".

The GWCT argued that there was a need to better understand how predation was affecting the long-term survival of capercaillie. "We take this threat to a splendid iconic Scottish bird very seriously and are discussing what form this research might take," a spokeswoman said.