Nearly all the UK's major supermarkets sell endangered and threatened varieties of fish, according to a report out today.
Asda has the worst seafood policies of the nine food chains included in the Greenpeace study, the campaign group said.
At least 13 species sold by the US-owned chain are on the Greenpeace danger list of fish which are over-exploited or destructively caught.
The Recipe for Disaster report awards the supermarket just one point out of 20 for its seafood policies.
Morrisons came second from bottom with two points out of 20, while Iceland came third from bottom with three points.
Greenpeace said Asda failed to provide it with any policy on selling sustainable seafood.
The campaign group assessed each supermarket chain according to a set of four criteria. These were: the number of destructively-fished species on sale; seafood procurement policies; support for sustainability initiatives; labelling and promotion polices.
M&S came out top of the league with a score of 17 points out of 20, followed by Waitrose with 15.
Seafood sales in the UK are worth £1.8 billion per year, with nearly 90 per cent of sales made through supermarkets, according to the report.
Greenpeace says supermarkets' purchasing power can influence the way the fishing industry operates. It wants stores to remove the most destructively-fished species from their shelves and adopt a sustainable seafood procurement policy.
Responding to the report, an Asda spokesman said the chain shared the same fish supplier as Sainsbury's.
"We're disappointed that Greenpeace has singled us out. We've met them time and again recently to explain that our approach to sourcing fish is no different to that of our competitors," he said.
Morrisons said in a statement that its fish were clearly labelled to promote consumer choice. The chain does not sell certain types of fish.