Plans to open up a scenic area in Central Scotland to more visitors have been thrown into doubt.
SNH said it did not want to scupper the Loch Leven project
Campaigners for a new pathway around Loch Leven, Scotland's biggest lowland loch, have clashed with a government agency intent on protecting birdlife.
In a late intervention, Scottish Natural Heritage said it wanted changes to the scheme, which has been approved.
But the project organisers have said that much of the funding depended on it getting under way soon.
The Rural Access Committee of Kinross-shire, the local group behind the project, also claimed the route changes and extra protection measures which SNH was seeking would cost an extra £150,000 which it cannot afford.
The 13.5km trail for pedestrians and cyclists around the side of the loch, a Special Protection Area for birds, aims to provide proper public access for the first time.
As well as being known for the island where Mary Queen of Scots was imprisoned, Loch Leven is also famous for its wildlife.
Work on the £1.5m, three-year construction project is due to start next month and its organisers said some of the grant money it won was conditional to work starting soon and completed in its entirety.
SNH has insisted it is not out to scupper the project, but said the extra precautions, such as high banks to screen passers-by from birds, were needed to protect important wildlife.
Path project director Neil Kilpatrick said the access committee also shared the concerns.
But he said of the extra measures: "They could actually stop the project altogether.
"If we cannot get the complete funding together then we'll not be able to do the project."
SNH eastern area director Andrew Bachell said the agency regretted having to make the last minute intervention.
"We've always had a concern that the access can only go in without disturbing the birds and there are ways in which that can be done," he said.
"We've worked with the organisation to find ways to stop disturbance of the wild foul interests of the nature reserve.
"But it's far better that we put that in place now than in two or three years' time when people say 'why didn't you protect the interests of the place'."
Mr Bachell said SNH had offered £220,000 to the access group to assist with the project, subject to agreement on mitigation and maintenance of the trail.
He added: "While agreement has been reached over most areas, in two locations it has proved more difficult than originally thought to specify measures which will both prevent disturbance and allow users to fully enjoy their experience of the loch, its wildlife and cultural and historic heritage."
He said the organisation was taking advice to resolve the issues as quickly as possible.