Iceland has been accused of threatening the long-term sustainability of vital mackerel stocks after unilaterally increasing its catches in the international waters of the north-east Atlantic.

In a damning leaked document agreed at a meeting in London in October, the EU, Norway and the Faroe Islands lambasted Reykjavik’s decision to significantly raise its quota without consultation. Russia and Greenland were also criticised.

“The delegations deeply regret the decision of Iceland in 2019 to increase its unilateral quota to levels well in excess of its previous claims, which are disputed by the delegations,” the document states.

“Such action, which has no scientific justification, undermines the efforts made by the EU, Norway and the Faroe Islands to promote long-term sustainability of the stock and the decision not to revise upwards the total allowable catch in 2019.”

The three said they “further regret that Iceland chose not to engage with its international partners” before “substantially increasing” its catch, and criticised similar, though less significant, unilateral quota increases by Russia and Greenland.

The escalating dispute echoes the cod wars of the 1950s, 60s and 70s, when the UK and Iceland clashed repeatedly and sometimes violently over Reykjavik’s ultimately successful efforts to restrict access to its rich fisheries by expanding the limits of its national waters, or exclusive economic zone.

Mackerel is the UK fishing industry’s most important stock, worth more than half a billion pounds a year. Historically, it has been shared with other coastal states whose waters the mackerel pass through – notably the EU, Norway and the Faroe Islands.

Each year, independent scientists at the International Committee for Exploration of the Sea recommend the total catch levels – generally about 1m tonnes – that can be safely taken in national and international waters to protect the stock’s health.

The North East Atlantic Fisheries Commission (NEAFC), made up of the EU, Norway and Denmark – representing the Faroe Islands and Greenland – plus Iceland and Russia, is then supposed to manage and enforce that limit.

Iceland, Russia and Greenland, however, do not agree quotas and in effect set their own limits, leaving the remaining members to cut their annual catch by about 15% to offset the activities of the other three, particularly in international waters.

That high seas catch by the NEAFC nations as a whole has soared from about 70,000 tonnes five years ago, or 9% of the total, to more than 200,000 tonnes, or more than 20% in 2018. In that year, 60% of the mackerel caught by Iceland came from international waters, according to NEAFC’s figures.

“If Iceland, Russia and Greenland continue to grow their catches at this rate, this now healthy stock could once again be seriously put at risk,” said Terri Portmann, a marine consultant, who said she was also struck by recent Icelandic trawler orders.

“In the past, Iceland, Russia and Greenland were happy to buy secondhand vessels from Scotland or Norway for this kind of fishing,” she said. “But they now have state-of-the-art, £30m-40m supertrawlers on order – huge investments for a fishery that these fleets have no agreed ‘allocation’ to fish for.”

Large UK fish retailers, including the big supermarket chains, were due to meet in London this week in part to discuss what action to take about the surge in unregulated mackerel catch, Portmann said, adding that “a good proportion” of it risked ending up in British supermarkets.

Charles Clover, the executive director of the sustainable fishing organisation Blue Marine Foundation, said fishing activity in international waters was increasing at an alarming rate and could put “an enormously important fish stock” at risk.

“The majority of this excessive fishing is conducted by countries not signed up to any overall allocation of the stock in the north-east Atlantic,” he said.

“This leaves the UK, EU, Norway and Faroe Islands having to second-guess what others may catch, and reduce their own fishing to maintain good stock health.”

This represented an “unfair distribution of a burden that should be shouldered by all”, Clover said, adding that the “logical and sensible course” would be to limit the mackerel catch that can be caught outside countries’ own waters to 10% of the total.

Iceland has said it will not back down, despite the threat this summer of EU sanctions if it does not stop unilaterally increasing its mackerel quota. Reykjavik intends to increase its catch from 108,000 to 140,000 tonnes, and Greenland aims to up its share by 18% to just over 70,000 tonnes.

The Icelandic goverment – which is investigating separate allegations that the country’s largest fishing company paid more than £6m in bribes to trawl for mackerel off Namibia – has said its fishing in northern waters is justified and responsible and claimed it has been kept out of negotiations with the EU, Norway and the Faroe Islands making all decisions.

“Fishing more than is advised by scientists is a serious matter but the responsibility can’t be shouldered entirely by Iceland,” the ministry said, calling for meaningful talks to resolve the row.

A fisheries official, Kristján Freyr Helgason, said the 15.6% of the overall mackerel catch left for Iceland, Russia and Greenland was “nowhere near enough”.

• This article was amended on 29 November 2019 to emphasise, for avoidance of doubt, that the 200,000 high-seas catch figure for 2018 encompassed NEAFC members as a whole. A reference to the Icelandic fisheries ministry should have said fisheries department.