There are fears about the effects on wild salmon

Pan Fish Scotland, which runs the Vuia Mor site in West Loch Roag, Lewis, said it regretted the incident.

It said two large holes in a salmon cage were probably made by seals.

The Association of Salmon Boards and the Rivers and Fisheries Trusts of Scotland have been preparing a formal complaint to the European Commission.

They said it was the fourth such escape in the islands since May and voiced concern that escaped fish could jeopardise the genetic integrity and survival of wild salmon.

One escape is one too many, and our company has a zero tolerance attitude to escapes Odd Geir Oddsen Pan Fish managing director

The latest incident is thought to have happened on 11 September.

Pan Fish managing director Odd Geir Oddsen said: "We greatly regret this incident and are obviously very disappointed that this should have happened at this stage.

"One escape is one too many, and our company has a zero tolerance attitude to escapes."

He said two large holes were found in the cage and preliminary investigations suggested it was caused by one or more seals.

Pan Fish said it had allocated a "substantial budget" to installing seal-scarers on every cage on sites in Skye and Mull, which the company said had "suffered greatly" from seal attacks.

In February, the Scottish Salmon Producers' Organisation (SSPO) said the number of salmon escaping from Scottish fish farms into the wild had halved.

It told a ministerial group that 157,000 fish escaped last year compared with about 300,000 in 2002.