Holidaymakers heading for the beaches of Northern France this summer have been warned about a green tide of potentially toxic sea lettuce that may have killed two men in under a week.

The growing public health scandal saw an 18-year-old oyster farmer die in Morlaix Bay last Saturday, and a 70-year-old retiree in Douarnenez Bay on Tuesday.

The Brittany beauty spots where the unidentified men apparently succumbed to heart attacks ‘in minutes’ are hugely popular with tourists, including thousands who arrive from the UK at this time of year.

Jean-Philippe Récappé, the Brest Prosecutor, confirmed that fears had been raised about possible Hydrogen sulphide poisoning caused by decomposing sea lettuce, or Ulva lactuca.

Six beaches in the area are currently closed because of the green algae that produces a smell like rotting eggs.

The colourless gas can attack nervous and respiratory systems inside the human body, paralysing breathing and then causing death.

Commenting on the death of the apparently healthy teenager at Morlaix, Mr Récappé said: "The cause of death has not yet been determined since the results of toxicological and pathological analysis is not yet known."

Mr Récappé said all visitors to the beaches needed to be aware of the potential dangers caused by the algae.