Officers from the Gangmasters Licensing Authority (GLA) joined police, health officials and fisheries authorities in Operation Enterprise; which involved undercover surveillance and night patrols on Rustington beach, West Sussex, and a 4.00 am raid at Billingsgate market.

The market supplies fishmongers, fish and chip shops, restaurants and cafés, London embassies and buyers for many of the capital's most famous department stores and hotels.

The GLA said there were concerns that clams were being harvested from an unclassified beach where there were concerns about E. coli contamination.

Crates of clams were taken away from the East London market and tests are being carried out on grit in the shellfish to see if they match the sands at Rustington and to determine if any clams were infected.

The Billingsgate raid was followed by the arrest of four men and the seizure of 50kg of clams.

The investigation began after complaints from residents about men working round the clock, digging for clams and cockles. There were concerns about damage to the beach and fears that the shellfish were sold for food despite possible E. coli contamination.

Andy Orpin, of Sussex Police said: "We talked to the men on the beach who all said they were digging the clams for use as fish bait, but they were taking large amounts of clams away, leaving big holes."