The National Trust for Scotland has stepped up its opposition to a giant new open-pen fish farm off Canna.

In  a formal letter to Highlands Council, the conservation charity declared that it had "no confidence the plans would not undermine, if not destroy, the very things that make Canna so important".

The Herald last week reported that the NTS had "significant concerns" over the proposed development, by Mowi.

The charity has expanded on those in its letter, describing the farm's site as biologically “the richest part of the Small Isles Marine Protected Area.”

It said the giant pens would prodice as much organic waste - mostly poo - as a town of 33,000 people, bigger, for example, than Oban. And it argued that Mowi had underestimated the number of boats and cruise ships that keep the tiny Canna economy ticking over.

The NTS was bequeathed the islands of Canna and Sanday in 1981 by the the Gaelic folklorist and scholar John Lorne Campbell, in 1981. Canna, which is four miles long and has a population of 18, who run the island under a deal with the charity.

READ MORE: National Trust for Scotland's major concerns over salmon farm plan for Canna

Stuart Brooks, the trust’s head of conservation and policy, in the formal letter argued the development would betray the wishes of John Lorne Campbell.

He said: “The protection of the cultural and natural heritage of Canna on behalf of the nation is our responsibility.

“Based on the information provided by Mowi so far about their fish farm proposals, we have no confidence their plans would not undermine, if not destroy, the very things that make Canna so important.

“In all conscience, we cannot support Mowi’s proposals. We would be failing in our core purpose as a conservation charity if we did, as well as betraying the wishes of John Lorne Campbell, who donated Canna and Sanday to our care.”

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A spokesman for Mowi said: “We are disappointed to see the NTS publicly campaign so early in this scoping process. Our science experts will continue to work with the Canna Development Trust to explore the potential for a sustainable aquaculture development, and look forward to discussing scientific evidence that is the basis for Canna’s decision whether to proceed.”