French police launched an investigation Friday after British Formula One driver Jenson Button and his wife Jessica were burgled on the French Riviera, the thieves making off with her engagement ring.

The couple were in a rented villa in the glitzy resort of Saint-Tropez with friends when the robbery happened on Monday evening.

"Two men broke into the property whilst they all slept and stole a number of items of jewellery including, most upsettingly, Jessica's engagement ring," a spokesman for Button said.

The thieves may have operated by pumping sleeping gas into the house through the air conditioning, he added.

"Whilst unharmed everyone involved is unsurprisingly shaken by the events," the spokesman added.

However, a source close to the enquiry played down the idea that gas was used.

"The investigation is under way. We're not ruling out any theory and we will carry out tests to see if any drugs were used," said this source, who wished to remain anonymous.

"We have not seem this modus operandi in this region and it certainly is not a growing problem," added this source.

Button's spokesman had previously said that gassing residents to steal their belongings had becoming an increasingly common crime in the area.

British newspaper The Sun reported that valuables worth £300,000 ($465,000, 426,000 euros) were stolen in the raid.

Button, 35, is currently signed to team McLaren and lives in Monaco, along the Mediterranean coast from Saint-Tropez.
Source: AFP