The international trade court has effectively outlawed the sale of dolphin-friendly canned tuna in American supermarkets, ruling such labels were unfair to Mexican fishermen.

The ruling, delivered on Wednesday, was the third from the World Trade Organisation against the use of a voluntary system of labels for dolphin protection and was immediately denounced by conservation groups.

‘It’s an absurd decision,’ said Mark Palmer, a marine mammal expert at the Earth Island Institute which devised the voluntary standard for canned tuna.

The label system was introduced 20 years ago to protect dolphins in the eastern tropical Pacific, the source for almost all of America’s tuna.

Campaigners say the labels have been successful in reducing the number of dolphins killed by tuna fishing fleets, and the system has strong support from the Obama administration and from congress.

But the WTO said in its ruling that the labels were not ‘even-handed’ when it came to Mexican fishermen.

‘We find that the US ‘dolphin-safe’ labelling provisions provide ‘less favourable treatment’ to Mexican tuna products than that accorded to tuna products of the United States and tuna products originating in other countries,’ it said.

A spokesman for the US trade representative, said the government stood by the labelling system. ‘The US remains […]