First it was killer sharks. Then it was killer bees. Now American holidaymakers heading for the beach this summer are being warned to watch out for another lethal hazard: killer sand castles.

The US bucket and spade brigade went on full alert yesterday after research by a top physician revealed that people falling into holes dug in the sand had accounted for more fatalities in the US since 1990 than shark attacks - 16 as opposed to 12.

The article, written by Dr Bradley Maron in the New England Journal of Medicine, said sand holes and tunnels, the byproduct of building sand castles and other juvenile beach fortifications, could turn into deathtraps with horrifying speed.

Although such incidents were extremely rare, Dennis Arnold, who runs a beach patrol at Martha's Vineyard, off Cape Cod in Massachusetts, said lifeguards were under orders to stop children digging deep holes. Occasionally some parents protest, he said. "They'll say 'You're ruining my kid's day!' and I say 'I don't care!'," Mr Arnold was quoted as saying.