Icelandic fishing boats have landed 108,000 tonnes of mackerel
The Scottish Government has described as a "scandal" the massive overfishing of mackerel that has been carried out by Icelandic boats.
BBC Scotland has learned that the Icelandic industry took five time its allowance of the fish from one section of sea this year.
Iceland told recent quota setting talks that the extra mackerel were a "by-catch" of herring fishing.
But Scottish fishing leaders accused Iceland of a "smash and grab" approach.
Quota settlements mean Scottish boats are allowed to catch more than half of all mackerel fished in the north east Atlantic in a deal which is worth tens of millions of pounds to the country's economy.
Other nations get smaller rights, with Icelandic boats allowed to take 20,000 tonnes of the fish.
But at the quota talks earlier this month, Iceland admitted it had actually taken 108,000 tonnes.
A spokesman for the Scottish Government told BBC Scotland the revelation was a "scandal."
He added: "At a time when the Scottish industry is at the very forefront of responsible and sustainable fishing practices in this sector, such activities by Icelandic fishermen threaten the long term sustainability of the mackerel stock.
"Fisheries Secretary Richard Lochhead has taken several steps to ensure this matter is raised at the very highest level.
"The Norwegian Secretary of State has also given assurances to us that they are to demand action by the Icelandic authorities to resolve this situation.
"We continue to keep this high on the agenda and ensure that the EU negotiators take an extremely robust line on this matter in discussions with Icelandic officials on an activity that cannot and should not be allowed to continue."
The Scottish Fisheries Federation said Iceland was taking s "smash and grab approach" to stock management.
Derek Duthie, secretary of the Scottish Pelagic Fishermen's Association, added: "We can't have fishing nations sticking two fingers up to the European agreement."