Dr Shalini Kanagasingam, from the University of Lancashire, said: “If you use a higher percentage, especially if it’s not monitored professionally by a dentist, you could end up causing irreversible damage to a tooth and you could actually cause a chemical burn.”
Dr Kanagasingam, who oversaw the tests at the university’s school of medicine and dentistry, described the gels bought by the BBC as “extremely dangerous”.
She added: “I can absolutely understand why someone would want whiter teeth. It’s something people aspire to nowadays.
“But I would always recommend that the public go to their dentist first, because it could be just a matter of getting a good clean, scale and polish rather than opting for bleaching agents, and especially bleaching agents at such high levels.”
The BDA urged the government to launch an urgent crackdown.
A spokesperson said: “We’re horrified that so called ‘training’ is being offered via social media, and syringes of 50% concentration of hydrogen peroxide are being sold in car parks.
“The risk must seem worth the reward to these fraudsters.”
A government spokesman urged the public to “contact Citizens Advice or their local authority Trading Standards department” if they had concerns over teeth-whitening products for sale in their area.