Mackerel has been struck off a list of sustainable fish by British conversationists who warn that overfishing is depleting stocks.

Previously championed as a healthy and ethical option packed with the fatty acid Omega-3, the oily fish should now be eaten only occasionally and where possible from local sources, the Marine Conservation Society (MCS) said.

The conservation group has removed mackerel from its "fish to eat" list, recommending herring and sardine as alternatives.

Mackerel populations in the Atlantic have shifted northwest from Britain towards Iceland and the Faroe Islands, where they are being heavily fished, the group said.

"The stock has moved into Icelandic and Faroese waters, probably following their prey of small fish, crustaceans and squid," said Bernadette Clarke, fisheries officer at the MCS.

"As a result, both countries have begun to fish more mackerel than was previously agreed.

"The total catch is now far in excess of what has been scientifically recommended and previously agreed upon by all participating countries."

Along with mackerel, gunard has also been taken off the list because of concerns about population levels of the fish, which is traditionally swept up by vessels fishing for other species and is often thrown back into the sea.

Herring, coley and Dover sole from the English Channel, on the other hand, can be eaten with a clear conscience, while whiting from the Celtic Sea off the south coast of Ireland has been given the green light for the first time.