Burglars suspected of using sleeping gas to ensure they are not disturbed during break-ins on Sardinia's Costa Smeralda

Police in the billionaires' retreat of Porto Cervo on Sardinia's Costa Smeralda believe thieves who made off with €315,000 (£280,000) in cash and jewels used sleeping gas on their victims to ensure they were not disturbed during the break-in.

Similar robberies have been reported this summer in France and Spain.

The burglaries in Porto Cervo, which took place last week, were only disclosed by police on Tuesday. The thieves sneaked into the rented holiday villa of a Milanese pharmaceuticals tycoon and left with a haul worth around €300,000. The businessman's 42-year-old wife, her mother and their daughter were all in the house, along with a servant, but no one heard the burglars, even though they took the windows off their hinges to get in.

At the villa next door, two holidaymakers found a watch and €15,000 in cash missing. They told police they had woken up feeling weak and dazed.

In July, "gassing gangs" were reported to be targeting caravans and camper vans in France. Thieves sprayed sleeping gas in through air vents before breaking in.

Earlier this month, at least six houses on an estate at Rincón de la Victoria on Spain's Costa del Sol were burgled by thieves thought to have used sleeping gas. One of the residents, José Luis Gómez, was quoted as saying the victims had woken "dizzy, with headaches, vomiting and stinging throats".

Porto Cervo was built in the 1960s by Prince Karim Aga Khan and it has long been a playground for the super-rich. Earlier this month,the sign at the entrance to the Costa Smeralda was altered, apparently by an insufficiently prosperous holidaymaker armed with a spray can. The "Smeralda" was deleted and replaced with the word "troppo", so it now reads in Italian: "Costs too much."