A Norwegian company has drawn up plans for Britain's first deep-sea salmon farm in waters off Shetland, The Ferret can reveal, prompting concerns it is an "experiment in factory farming" that could damage the environment.

Offshore salmon farming involves raising fish at sea in submerged cages, several miles away from coastal areas. Modelled on oil platforms, these super-sized farms are bigger than inshore sites and more robust for deeper waters.

The new technology is viewed by supporters as a more sustainable form of fish production. But critics have described offshore farming as "factory farming of the sea" and super farms off Norway have suffered problems such as fish escapes which can impact the marine ecosystem.

The finding comes from government documents released under freedom of information law to The Ferret, along with plans leaked from Norwegian aquaculture giant SalMar. They show that the Shetland site, if sanctioned, would see three huge farms spread across an area several times the size of Lerwick - rearing more than five million fish.

Proposals for the project are already developed enough for Crown Estate Scotland to be considering specific coordinates for the farms. The plans have also been shared with the Scottish Environmental Protection Agency, Shetland Islands Council, and the Scottish Government.

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MSPs were told last month that while firms operating in Norway with links to Scotland are looking at offshore opportunities, no companies currently operating in Scotland are considering this.

Campaign group Animal Equality criticised what it described as a "veil of secrecy" over SalMar's plans, while environmental charity Green Britain Foundation (GBF) said the aquaculture industry was demonstrating it "can't be trusted".

"They're apparently planning massive offshore operations in Shetland waters," said GBF's founder Dale Vince. "We don't need another experiment in factory farming, in a pristine environment, from an industry in pursuit of profit at any cost."

SalMar did not respond to our request for a comment. Salmon Scotland - which represents salmon farmers - said it had no knowledge of any company plans to move immediately into offshore locations.

SalMar in Shetland

During a visit to Shetland in January, SalMar presented a plan to locals for three separate farms, each five kilometers apart and rearing 1.6 million fish apiece. The site's total footprint would cover more than 25 square kilometres.

The plans said the super farm could bring dozens of jobs to Shetland. The scale of the project, however, has concerned local fishers.

"It will mean spatial squeeze inshore and offshore for us," said Sheila Keith, of Shetland Fishermen's Association. "I'm not convinced growth is always the best thing for industries in Shetland when we have finite space and parameters to work with. Is expansion the best thing for Shetland?"

We don't need another experiment in factory farming, in a pristine environment, from an industry in pursuit of profit at any cost.

Dale Vince, Green Britain Foundation.

SalMar presented its plans to a number of local stakeholders, including Yell community council at a public meeting. The community council declined to share the presentation with The Ferret but did volunteer emails from SalMar in which the company said: "We have chosen not to share any written material such as the presentation shared with you and would not like this to be shared with externals such as the media, please do not share it".

The firm raised similar concerns with government officials. In emails discussing freedom of information requests, SalMar urged officials to withhold details and they agreed. "We will redact the specifics on proposed and the more specific area of interest first identified, as well as the reference to as previously discussed," an official wrote to SalMar in March.

Shetland Islands Council rejected a similar freedom of information request for emails exchanged with Sa...