Reports of illegal teeth-whitening that could leave patients at risk of health problems including burns or lost teeth have increased, the BBC has found.
General Dental Council (GDC) figures showed a 26% rise in reports last year.
Teeth-whitening can only be performed legally in the UK by professionals registered with the GDC.
One beauty school claimed to have provided "thousands" of candidates with illegitimate qualifications, an undercover investigation found.
Failure to comply with the requirement to be registered can result in a criminal record and an unlimited fine.
Untrained beauticians using teeth-whitening kits have been known to cause tooth loss, burns and blisters.
Dr Ben Atkins, president of the Oral Health Foundation, said: "When things go wrong in dentistry, they can really go wrong.
"I've been that dentist with the full back up service when the patient's had that heart attack.
"It would be catastrophic for the patient and the person who's been trained and told it's legal to do it."
Last year 732 cases of illegal-teeth whitening were reported to the GDC, a 26% increase from 582 in 2018 - though the figure was higher in 2016.
The dentists' regulatory body relies on reports from customers, so the real number could be much higher.
The GDC said it had launched 126 prosecutions against illegal teeth whiteners since 2015.
It lacks the powers to prosecute those performing training, such as those filmed by BBC London.
The BBC uncovered several companies offering just a few hours of training for fraudulent qualifications.
Two undercover BBC researchers attended a course with the London School of Nails and Beauty that lasted five hours.
Attendees were told they could earn "from £80 upwards per session" after receiving a certificate, which they were told would allow them to set up their own business to treat customers.
School principal Cha McDonald said the process was "legal" as customers would be asked to carry out parts of the procedure themselves.
She claimed "thousands" of people had undergone training at the school.
A spokesperson for the General Dental Council said: "Handing an individual a tooth whitening tray and advising them on application, amongst other things, could constitute the giving of 'advice or attendance' and in those circumstances would be a criminal offence."
Emergencies were not covered at all during the training attended by the BBC and when asked what to do in an emergency, Ms McDonald advised the BBC researchers posing as students to "call an ambulance like everybody else".
The BBC asked the London School of Nails and Beauty why it was providing training that could open candidates to prosecution but there was no response.
The British Dental Association (BDA) said "sham" schools preyed "on vulnerabilities of beauticians and others".
People attending these schools were putting themselves at risk of ending up in jail or being fined, according to Dr Len D'Cruz from the BDA.
One beautician who had undergone similar training said: "I may as well have burnt the money I spent.
"I was struggling as a beautician as it was and I nearly went bankrupt.
"I think as a direct result I lost all my confidence. I went through anxiety and suffered depression. It's disgusting people like me are being misled."
This story will be featured on BBC London TV News and Inside Out on BBC One in London on Monday 10 February at 19:30 GMT and afterwards on BBC iPlayer.