Salmon, grown by a Scottish fish farm, were altered so that they grew four times faster than normal

The tests started in 1996 and lasted about 18 months with the blessing of a number of government departments under the Conservative administration.

The issue re-emerged in the House of Commons when Labour MP John Ruddock asked Scottish Secretary John Reid to reveal details of the tests.

Mr Reid told the MP 50 fish were grown during the study and were then destroyed.

Scottish Secretary Dr John Reid: "Experimentation under strict controls"

He said: "Copies of a growth hormone gene from taken from Chinook salmon were introduced into 10,000 Atlantic salmon eggs.

"The fish were grown in a land-based containment facility for up to one year. Approximately 50 of the fish grew at four times the normal rate, with no sign of abnormalities."

Amid fresh concerns over the tests, Mr Reid told BBC News: "These were commercial experimentations, held under strict control, with no government money and started after the last government allowed them.

John Reid: Explained tests

"The tests are long finished, all the salmon were destroyed and there is no question of any safety problems arising."

And Mr Reid said the Health and Safety Executive is notified of experiments such as those carried out in 1996.

The privately funded research was carried out by Otter Ferry Salmon, on Loch Fyne on Scotland's west coast.

A Canadian company approached the firm with the idea for developing fast growing fish - reared in tanks on land under strictly controlled conditions.

When the tests were approved they attracted a high profile, prompting the Scottish Salmon Growers' Association to distance itself from them.

Green MSP Robin Harper: "This fish could do untold damage."

"They were very public in July 1996," said growers' association Chief Executive William Crowe.

A policy was drawn up by the association, claiming that "in accordance with sound environmental practice" transgenic salmon production would be totally rejected.

London backbench MP Joan Ruddock tabled the question on the tests after hearing "persistent rumours" about GM fish studies.

Joan Ruddock: "Asking more questions"

The transgenic salmon proved fish can grow to market size in 12 to 18 months rather than three years.

That could double the turnover of salmon raised in tanks - and potentially double the profit.

Ms Ruddock said: "The results were obviously what they were looking for. If you can cut the costs of production and get the fish to market faster, you can maximise profits.

"That's obviously the route that many companies are trying to go down. It's the same argument for increased yields with genetically-modified crops."

Scottish Green MSP Robin Harper warned of the potential consequences of the research.

Untold damage

He said: "This fish, if it escaped into the north Atlantic environment could do untold damage to the ecology both of the north Atlantic and Scottish salmon rivers."

David Patterson, technical director of Otter Ferry Seafish, as the company is now known, said there were fears at the time that the Norwegians could adopt the technology.

He said the Scottish Office was worried about domestic producers losing their competitive edge to countries employing the technique.