Untrained salon staff are carrying out teeth whitening on clients, according to consumer magazine Which?
It is legal for people to use a teeth whitening kit at home
The General Dental Council (GDC) said it was illegal for non-dentists to carry out the procedure.
The dental regulatory body has launched investigations into six London clinics offering the service and visited by Which? magazine.
Habia, the body which helps salons understand legislation, said some may have been confused by regulations.
James Goonik, of the British Academy of Cosmetic Dentistry, said the use of a substance by three of the clinics - chlorine dioxide - was particularly worrying.
"This chemical is a bleaching agent used to purify water and is very acidic," he said. "It can wreck tooth enamel".
The other three clinics reportedly used a hydrogen peroxide formula at a strength above the legal limit.
Concerns were also raised about the failure of some of the clinics to check for gum disease.
Clinics saying they would take impressions of teeth to check for problems were accused of acting illegally because this would have been carried out by an untrained person.
But while it may be illegal for a non-dentist to carry out teeth whitening on someone it is perfectly legal for someone to take home a teeth whitening kit and perform it on themselves.
Wendy Nixon, of Habia - the government-approved body which helps salons understand legislation - said it was this which may have confused salons, most of which "were not deliberately acting irresponsibly".
"A lot of people just don't know about these regulations and they are continually being pressured by very pushy sales people who say it's perfectly OK for them to offer this service," she said.
"At the same time, there's more and more demand from clients."
Whitening has become a fad of recent years, popularised in part by celebrities.
However, the desire for gleaming teeth is not a modern phenomenon. The Romans, for instance, used urine to whiten theirs.