The body which certifies that fish have been caught sustainably has been accused of "duping" consumers by giving its eco-label to fisheries where stocks are tumbling.

The Marine Stewardship Council (MSC) manages the labelling system that tells consumers which species of fish they can buy safe in the knowledge they aren't destroying stocks.

It recently celebrated the 100th award of its eco-label – to the Barents Sea cod fishery – but a series of decisions allowing controversial fisheries to be granted the prized MSC label has prompted severe criticism of the organisation.

Richard Page, a Greenpeace oceans campaigner, said decisions to certify some fisheries "seriously undermine" the MSC's credibility.

"I will go as far as to say consumers are being duped. They think they are buying fish that are sustainable and can eat them with a clean conscience."

Among the most controversial rulings is the award of an MSC label to the Ross Sea Antarctic toothfish fishery which is still regarded by scientists and the industry as an exploratory fishery. The species is so little understood that researchers still do not know even basics such as where the fish spawns.

Others include krill in the Antarctic, , tuna and swordfish off the US coast, pollock in the Eastern Bering Sea where stock levels fell 64% between 2004 and 2009, and Pacific hake which suffered an 89% fall in biomass since 1989.

Chris Pincetich, a marine biologist with the Turtle Island Restoration Network, said: "The MSC has rushed to accept applications from hundreds of fisheries around the globe in order to grow their business and network. Many of those are actually viewed by scientists as unsustainable. They should really take a closer look before they even engage with those fisheries."

Page said of the toothfish decision: "It should never have been up for certification in the first place. There just isn't sufficient information to say whether it's sustainable or not."

James Simpson, of the MSC, insisted all assessments were "scientifically robust" and said they are designed to ensure the "biological and ecological" components of each fishery are not compromised. Moreover, it introduces better levels of protection to stock levels than might otherwise be found in fisheries.

He recognised, however, that "for some of our critics, the MSC test of sustainability is not high enough" and said: "Behind the controversies there is evidence of real environmental benefits occurring – many of them driven by fisheries' desire to attain and keep their MSC certificates.