France's oysters have been struck by a mystery plague that has killed millions of the prized shellfish and plunged the industry into crisis for the second year running.

Scientists have yet properly to determine what has caused up to 90 per cent of baby and juvenile oysters, due to be eaten by Christmas 2010, to have died.

Producers in Normandy are so worried that last month they handed out free boxes of the shellfish near Caen chanting: "Take these oysters, they may be the last you'll ever eat."

The deaths have come in two waves. The first, in May, hit the Mediterranean - including Corsica and the Etang de Thau, a salt-water lake near Montpellier - and also the west coast in the bay of Arcachon. The second struck oyster farmers all the way as far as Normandy.

"It's a catastrophe," said Goulven Brest, a producer from northern Brittany, home of the famous flat oysters of Cancale.

Last year, France's oyster industry - Europe's largest - was hit by its worst crisis since the native European or "Portuguese" oyster was all but wiped out 30 years ago. Since then almost all oyster farms in Europe have been restocked with the Pacific "creuse" oyster from Japan and British Columbia.

"This year is different, as it has come in two waves," said Sebastien Chantereau, from the national shellfish producers' body, the Comité National de la Conchyliculture. "Although each was less violent than the one that struck last July, together the damage is roughly the same. If we have one more wave between now and September it will be a total catastrophe," he told the Daily Telegraph.

Experts have no idea why oysters in some parcs, or beds, have come out unscathed while others nearby have been completed wiped out.

Scientists at Ifremer, the French Institute for Exploitation of the Sea, have a number of theories to explain the plague. They believe the combination of a virus and up to three types of bacteria may play a role.

Warmer sea temperatures, perhaps related to global warming, may also be partially responsible, as they weaken the young oysters and increase the amount of microscopic plankton the baby shellfish eat so they gorge themselves to death. Toxic algae and chemical residues could also be factors.

It took scientists almost a decade to work out why France's oysters died out in 1970. Producers say they cannot afford to wait that long.

"We'll certainly determine the cause one day, but if we wait to know the whys and hows of these deaths, we'll no longer be there. We need to find an alternative between now and then," said Mr Brest. "Some producers will have no oysters left by next spring," he warned.

The farmers are asking the government to provide emergency funds on a similar scale to last year, when it gave £32 million in assistance.

Scientists are looking at bringing in more resistant strains of the species from Japan or Australia, taking genetically stronger variants, or trying out a new species altogether.

The only silver lining is that the overall baby oyster population is considerably higher this year, so more individual molluscs have survived.

Producers say the death of baby oysters should not put people off from eating mature shellfish, as they are unaffected and pose no health risk to humans.