Up to five tonnes of cocaine seems to have been washed up along the Atlantic beaches of France and Spain, apparently a single shipment dumped overboard by Mexican smugglers.

"Its a serious problem. The street price has fallen by over a third, there's been a big rash of related hospitalizations, and a large number of people who had never even tried cocaine are now hooked," said Jean-Michel Delile of the Bordeaux drug research center.

The 1kg bricks are shrink-wrapped and bear a crown logo and the words Calidad Total (Total Quality). They have appeared on the French coast between Arcachon and Biarritz, and Spanish beaches as far west as Bilbao.

Since January the French and Spanish police have recovered 1,500kg, tested as 85% pure. "But what we've seized can only be a small fraction," a police spokesman said in Hendaye, a Basque resort on the border. "We can't put men along every metre of coast."

Analysis shows the cocaine almost certainly comes from Mexico; it could have been dumped in panic at the approach of a patrol vessel, or lost while being transferred from a freighter in heavy seas.

The trove was a temptation, the spokesman said: "People discovered a previously unsuspected passion for strolling along the beach at midnight."