Anaesthetic gas may have been used against Jenson Button and his wife Jessica during a burglary in France, the Formula 1 driver's spokesman says.
Two men, who stole jewellery including Jessica's engagement ring, may have pumped gas through the air conditioning before Monday's break-in, he said.
The spokesman said they were unharmed but "unsurprisingly shaken".
But medical experts said they were sceptical that burglars had used gas in the raid on the villa in Saint Tropez.
The Sun newspaper reported that valuables worth £300,000 were stolen.
The couple, who married in December, were staying with friends in a rented villa at the time of the burglary.
Button's spokesman said: "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.
"The police have indicated that this has become a growing problem in the region with perpetrators going so far as to gas their proposed victims through the air conditioning units before breaking in."
Philippe Guemas, deputy prosecutor in Draguignan, France, told Europe1 radio that "nothing has been established" and said the claim that gas was used in the raid was made by Button's entourage as the driver felt unwell the following day.
"To our knowledge there has never been a burglary like this in St Tropez where gas was used to knock out the victims," he said.
"We have taken blood samples, which will be analysed."
An operations director for SRX, a home security firm based near Mr Button's home, said the possibility of burglars using gas did not surprise him "at all" and he had received requests to install measures to prevent such attacks.
But the Royal College of Anaesthetists said although it could not rule out that "some sort of agent" had been used, it was "highly unlikely" the group had been rendered unconscious by anaesthetic gas.
This, a spokesman said, would require "massive amounts of gas".
He added: "When you combine that with the fact that these gases are expensive and difficult to get hold of, we are very sceptical."
Hugh Schofield, BBC News
There has been an awful lot of rumour about sleeping gas burglaries in France, but precious little hard evidence.
A number of doubts spring to mind: What gas do they use? In what quantities? How do they ensure it will be effective in the right parts of the house? And through the air-conditioning system? Really?
In the camping-car community, there have been persistent claims about burglars using sleeping gas.
On chat-lines, holidaymakers share their experiences and ask about ways to protect themselves.
But again - no-one has ever appeared in court charged with gassing and stealing. No police officer is on the record.
Who knows? Maybe Jenson Button was indeed the target of a gas-and-steal.
The head of a security firm I spoke with in St Tropez says he has never heard of an actual case of a sleeping gas burglary. "I don't believe in it at all," is what he said.
Button, 35, who drives for team McLaren, is based in Monaco, about 80 miles (130km) along the coast from Saint-Tropez. He won the F1 championship in 2009 driving for Brawn GP and finished in eighth place in 2014.