A number of beauticians in London are performing illegal teeth-whitening procedures, which can leave patients in crippling pain if they go wrong.
One place was secretly filmed by BBC London using illegal levels of the hydrogen peroxide at a strength of 25%.
The maximum legal strength useable by a registered dentist in the UK is 6%. If used incorrectly it can cause tooth loss, burns and blisters.
The General Dental Council (GDC) has seen illegal treatments rise, it said.
Only dentists and registered professionals can do the procedures on customers with dentist prescriptions.
Whitening products available on the high street may contain up to 0.01% of the chemical.
Richard Grieveson said 35% hydrogen peroxide was used on him by a beautician not filmed by the BBC.
"It can only be described as a really, really, sharp excruciating pain. And it went on for a good 45 minutes," he said.
"A few days later my mouth became very raw and very sensitive. Basically it was like open sores."
'Lives at risk'
BBC London identified a number of places in the capital providing illegal teeth whitening treatments.
One was Plaistow beautician Fran Lowe, who was filmed using 25% hydrogen peroxide and admitted knowing at the time it was illegal to do so.
She charged £75 for the treatment and carried it out at home. She also offered teeth scraping – a procedure that should only be carried out by a dental professional.
In addition, Ms Lowe sold the reporter a tube of carbamide peroxide, a product which when used in the whitening procedure breaks down to 7% hydrogen peroxide.
She later denied using peroxide gels and claims she did so to obtain a sale. She told the BBC she had not known she was breaking the law and had stopped providing the treatment.
In another case, Paddington beautician Ashleigh Smith charged £100 for teeth whitening despite admitting she had only a day's training in the procedure.
She said the product she used did not contain peroxide and would not comment further.
Staff at the Aanu Beauty salon in Ilford said they thought it was legal for them to supervise the use of whitening gel, as long as they did not apply it themselves. The GDC says this is not the case.
Employees asked a reporter to self-administer by placing a gum shield with the product in their mouth. The salon later said it was not peroxide and told the BBC it no longer offered teeth whitening.
Dentist Ben Atkins said the footage of Ms Lowe was the worst example of illegal practice he had seen.
"It's putting patient lives at risk, doing dental procedures not in a dental surgery," he said.
The GDC declined to comment on specific cases.
Its head of illegal practice, Francesca Keen, said: "If you're seeing someone on the high street that's attended a one day course and putting that into perspective – dentists attend a five year training programme in order to qualify – you're putting yourself in harm's way."