Opponents of the 400 berth, 10 hectare marina at Gallows Point near Beaumaris, claimed it would damage the surrounding mussel beds, which are the largest in the UK.

Anglesey Council and the Crown Estate Commissioners, legal owners of the foreshore and seabed included in the fishery, argued that, subject to obtaining the necessary planning consents, they had the legal right to go ahead. The fishery was created in 1962 by a 60 year order, and giving his ruling at London's High Court, Mr Justice Davis said that the 1962 Order was lawful and the planned construction would be unlawful.

James Wilson, who with his brother Andrew leases land from seabed co-owners Anglesey Council, said: "We're very happy, obviously - it obviously removes something we consider to be a huge threat to our industry."

The brothers have been working on the fishery since taking over from their father in 1991, and the beds produce between 6,000 - 10,000 tonnes of shellfish each year - or 50 - 75 per cent of the total amount of shellfish on the UK market.

Mr Wilson added: "Everybody recognises that there is a great need for economic development on the Island - everybody on the Island supports the idea and are worried about the economy.

"However this development is a threat to the existing economy of mussel farming."

Afterwards, solicitor Andrew Oliver representing North Western and North Wales Sea Fisheries Committee, lessee Andrew Wilson and Extramussel Limited who, together with the Welsh Ministers, are fighting the proposal, said that they were "delighted" with the outcome.

In response, an Anglesey Council spokesperson said: "The decision was handed down by his Honour Justice Davis who finds in favour of the defendants on the three elements of the declaratory proceedings

"The council is disappointed with the decision and officers will be taking a report to the full council meeting on Friday (tomorrow) to consider the merits of appealing the decision."

Dafydd Jones, a director of marina company ABC, added: "We are surprised but do not believe this will be the end of the matter, and we are still totally committed towards the project."

The organisations behind the proposal now have 21 days to file an appeal, and then 14 days to lodge a skeleton argument.