Efforts to ban the practice of discarding edible fish at sea could be thwarted by a group of EU member states which are planning to block reforms aimed at conserving dwindling fish stocks.

Campaigners, including the chef Hugh Fearnley-Whittingstall, have been calling for a ban on discards – where as many as two-thirds of healthy fish caught by fleets are thrown back into the sea, dead, because they are less valuable than the species fishermen are trying to catch – for more than a year. In his series, Hugh's Fish Fight, he said half of all fish caught in the North Sea are thrown back. He called moves to block the proposed ban "worrying in the extreme".

The EU fisheries chief, Maria Damanaki, had backed a change to the European common fisheries policy (CFP) to ban discards. Her reforms would mean fishermen would be forced to land all fish they catch, in return for compensation.

But on Monday, some member states led by France and Spain will hijack a council meeting of the EU's fisheries ministers, the Guardian has learned. They will attempt to pass a "declaration" allowing discards to continue indefinitely – dismissing the ban as "unrealistic" and "too prescriptive".

Some fishermen – mainly in companies with industrial-scale vessels – want to keep the present arrangement because by throwing back lower value, though edible, fish they can maximise their profits.

If the declaration is passed, experts warned, the hopes of banning discards would effectively be over."This will kill the reform," one Brussels insider said. "It would be the end. Monday is make or break time for the policy." "This declaration looks like a vote for maintaining the status quo, or at best tinkering at the edges, and allowing hundreds of thousands of tonnes of perfectly edible fish to continue to be wasted in European waters," said Fearnley-Whittingstall. "If it succeeds I fear we can expect negligible progress on discards for many years ahead."

The declaration's signatories "restate their commitment to an ambitious reform of the common fisheries policy, [and] reiterate their view that the wasteful practice of discarding fish, that is tolerated and in some cases even promoted by the current management system, constitutes a considerable obstacle on the road to a sustainable fisheries policy".

But it adds: "[We] consider a discard ban as proposed in the draft basic regulation of the future common fisheries policy … is unrealistic and too prescriptive, and a pragmatic approach is needed especially in the context of mixed fisheries, particularly in the Mediterranean [and] support instead the inclusion of a significant reduction of discards … on a fisheries-based approach."

These caveats would effectively mean fishermen could continue to discard edible fish at sea indefinitely.

"This will kill the reform," one Brussels insider said. "It would be the end. Monday is make or break time for the policy."

Discarding results in as much as two-thirds of the fish caught being thrown back in the water, with about 1m tonnes estimated to be thrown back each year in the North Sea alone. Discarding is a consequence of the strict quotas in the EU under the common fisheries policy on the amount of fish that boats may land. When fishermen exceed their quota, or catch species of fish for which they do not have a quota, they must discard the excess.

Ruth Davis, chief policy adviser for Greenpeace UK, said: "This declaration shows that fishing ministers in some European countries will stop at nothing to slow down reform of the CFP. In doing so, they're defending the interests of a minority of extremely powerful, greedy, industrial-scale fishing companies at the expense of many thousands of sustainable and small-scale fishers."

The member states currently signed up to the declaration are led by France and Spain – the two countries "that make the music" on fisheries policy, according to one fisheries expert, as the positions those two adopt are usually followed by newer member states. They are joined by Portugal and Belgium, but other countries are wavering. Italy and Cyprus may sign up, as may the Irish. Heavy lobbying is going on behind the scenes, and will continue until the meeting on Monday.

Germany is undecided on its stance, but Richard Benyon, the UK fisheries minister, said: "We do not support this declaration and will pressing the EU hard to end the wasteful practice of discarding fish."

But many fishermen, particularly companies with large-scale industrial fishing vessels, would like to retain the practice, because it enables them to throw away lower value fish and keep the most valuable in order to maximise their profits.

Spain's stance was prefigured in a secret document revealed by the Guardian this year, which showed that the previous Spanish government was planning to scupper the proposed ban. The incoming government said at the time its position had not been decided, but it is now evident that Spain – which has the EU's biggest fleet and receives more of the EU's fishing subsidies than any other member state – is orchestrating opposition to the ban.

"Ending this horrendous waste has to be the number one priority of a reformed CFP, and I'm going to do all I can to keep it on the top of everyone's agenda," said Fearnley-Whittingstall, "Over three quarters of a million people have already signed the Fish Fight petition calling for an end to discards ... Now its time for the politicians and decision makers to make it happen."