Consumers of tuna from the world’s biggest fishery are are being “betrayed” over its sustainability, according to a coalition of scientists, retailers, politicians and campaigners, including Hugh Fearnley-Whittingstall.

The vast Western and Central Pacific fishery provides about half of the world’s skipjack tuna, the type most commonly found in cans on supermarket shelves. Some is certified as sustainably caught by the Marine Stewardship Council (MSC) and carries the group’s “blue tick” logo. But the same boats can also use, at other times, unsustainable methods to catch uncertified fish, a contradiction seen as unacceptable by the new On The Hook coalition.

“MSC are betraying the trust of consumers and duping them into purchasing what they believe is truly sustainable tuna,” said Prof Callum Roberts, a leading fisheries expert at the University of York and part of the coalition. Polling by the group showed that 77% of UK consumers who were aware of the MSC believe that vessels that caught MSC labelled products should meet MSC requirements at all times.

Roberts praised the MSC’s past work on tackling overfishing but likened the tuna issue to rewarding a criminal for the days on which he does not commit a burglary: “The situation is intolerable. The MSC has lost its moral compass.”

Fearnley-Whittingstall said: “I rely on the MSC to provide sustainable fish for my restaurants, and so do my customers, and if I am not convinced their certification is reliable I will have to reconsider. It’s time to see some real reform.”

Another coalition member and a former UK fisheries minister, Richard Benyon MP, said: “Like many, I have long held the MSC in very high regard. That reputation has been hard won around the world but it is squandered quickly. The MSC must urgently review their standard to prevent MSC-certified products being caught alongside unsustainable methods of fishing.”

The Western and Central Pacific fishery covers the waters of eight island nations, including Papua New Guinea, the Solomon Islands and the Marshall Islands. The use of nets to catch free swimming tuna in the region was certified sustainable by the MSC in 2011 and is set to be renewed this year, according to an independent technical report.

But the same vessels can also at other times use fish aggregating devices (FADs) – floating mats that attract life in the open ocean. The MSC’s website says: “It’s an easy way to fish, but it’s largely a wasteful and destructive one, as many young tuna are caught before they can reproduce, with other species ending up as bycatch.”

Roberts said: “These things are a magnet for all kinds of things, so there is a whole community of animals [including turtles and sharks] and all of these get caught when the net goes around it.”

Claire Nouvian, president of ocean conservation NGO, Bloom, said: “It is clear that MSC, by certifying certain parts of a fishery are creating an illusion of sustainability.”

The MSC said a strict chain of custody requirements ensures that certified and non-certified catch are clearly segregated. On the issue of whether fisheries should be allowed to catch both certified and non-certified fish, the MSC will launch a formal consultation on Friday, with a recommendation going to the MSC board in January.

However, standard practice means that fisheries are normally given three years to adapt to new standards, meaning the Western and Central Pacific tuna fishery is very likely to have a new five-year certification before any changes can be made.

“As a global not-for-profit, we remain committed to our vision for the world’s oceans to be teeming with life, and seafood supplies safeguarded for this and future generations,” said David Agnew, MSC science and standards director. “The MSC works tirelessly to understand and unite a complex set of often differing views among our stakeholders – fisheries, retailers, brands, NGOs, scientists, and of course consumers – and galvanise them all in pursuit of a shared goal that all can benefit from, regardless of their differences.”

Pacifical, the marketing company for the Western and Central Pacific tuna fishery tuna, defended its operation in a letter to the MSC on 29 August, in which it said managed FAD fishing could be sustainable and that it aimed aim to get MSC certification for this within five years.

Princes, a major tuna brand, sources MSC-certified tuna from the Western and Central Pacific fishery and a spokesman said: “MSC is widely recognised as the world’s most robust standard for sustainable fishing. As such, Princes fully supports the MSC and its certification programme.”