Some 14,300 tonnes of processed bluefin traded over the period 2000 to 2010, a figure obtained from official trade and customs databases, was the equivalent of 18,700 tonnes of live fish, it said in a statement.

If confirmed, the concealment would amount to illegal fishing and constitute an environmental crime, said the WWF, as it demanded a probe.

Panama is a main export route of bluefin to key market Japan.

"This is the first-ever study on this issue and it probably shows only the tip of the iceberg," Sergi Tudela, head of the WWF's Mediterranean fisheries programme said in a statement.

The organisation urged the International Commission for the Conservation of Atlantic Tunas (ICCAT) and the countries whose fleets are responsible for this state of affairs, to launch an urgent investigation.

They included Spain, Italy, Morocco, Tunisia and Turkey, all of whom were ICCAT contracting parties at the time, said the statement.

The total allowable catch today is 12,900 tonnes per year, and all trade in Atlantic bluefin tuna must be reported to allow cross-checking with quotas.

"According to available records, not a single shipment identified by the report was ever reported to ICCAT."

The WWF said the unreported transactions may have happened without the fish ever having reached Panama's shores -- the trade going though Panamanian intermediaries on the high seas.

The ICCAT did not respond to telephoned and emailed requests to comment.

Earlier this month, an ICCAT evaluation provided the first signs of an increase in Atlantic and Mediterranean bluefin stocks.

But the WWF and other environment groups urged the commission not to raise catch quotas at its next conference in Agadir, Morocco in November, saying the numbers were still fragile.

In September, experts warned at an International Union for Conservation of Nature (IUCN) conference that global tuna stocks were fast reaching the limits of fishing sustainability.

Five of the world's eight tuna species are classified as threatened or near-threatened, they said.

The Atlantic bluefin species, which can live to 40 years and grow to more than four metres (13 feet) long, is in the gravest danger of disappearing with stocks estimated to have halved over four decades in some areas.