Some home teeth-whitening kits sold online contain dangerous levels of bleaching chemical that can burn gums and cause tooth loss, an investigation has revealed.
The worst had more than 300 times the recommended amount of hydrogen peroxide, says consumer group Which?.
A further 20 products out of 36 tested also had illegal levels of the bleach.
All have since been removed from sale by outlets including AliExpress, Amazon and eBay.
Oral Orthodontic Materials Store, the company behind the product containing the highest levels of hydrogen peroxide - at 30.7% - told Which? that it had "found the mistake, corrected it in time" and that the product "has been removed from sale".
AliExpress, the online store selling it, said: "We take product safety very seriously and after being notified by Which? of its findings, we took prompt action and removed the third-party product listings identified to be in violation of our listing policy.
"We are a third-party marketplace and all merchants selling on AliExpress must comply with our platform rules and policies, as well as comply with all local laws and regulations. We will take action against sellers that are found to be in violation of our terms."
Products sold over the counter in the UK should contain no more than 0.1% hydrogen peroxide, while the legal limit for application by professional dentists is 6%.
Six of the products that Which? tested contained more than 10% hydrogen peroxide.
Which? says consumers should seek help from a professional dentist if they want whiter teeth, as the procedure, if done incorrectly, can cause lasting damage.
Dr Paul Woodhouse, a director of the British Dental Association, warned: "Hydrogen peroxide is a seriously strong chemical and not to be messed with. Dentists are trained in its usage and they also know what whitening products are effective to use and safe for teeth and gums.
"If you destroy gum tissue, you are never going to get it back and you lose your teeth. If it penetrates the surface of your tooth, which is likely, it's probably going to lead to the death of that tooth."
Teeth whitening - facts and tips
- Whitening isn't permanent. It can last from a few months to up to few years
- You can have it done on the NHS only if there's a medical justification
- Providers offering the service in the UK should be registered with the General Dental Council. Dental hygienists and therapists can carry out teeth whitening on the prescription of a dentist if they have the necessary additional skills
- Some beauty salons offer teeth whitening, but this is illegal if there's no dental professional present
If you go the DIY route, look to buy teeth-whitening kits from a reputable high-street retailer, as the legal responsibilities to ensure products are safe are stronger there than on online marketplaces, says Which?
It's shared the findings with the Office for Product Safety and Standards.
Sue Davies, head of consumer protection policy at Which?, said: "It's clear that self-regulation is not working, leaving people exposed to a flood of unsafe products online.
"It is absolutely crucial that online marketplaces are given greater legal responsibility for the safety of products sold on their sites, so that shoppers are far better protected from dangerous and illegal items."
A spokesperson for the Office for Product Safety and Standards said: "By law, cosmetics may be placed on the UK market only if they are safe. There are specific restrictions in place for products of this kind and we would urge anyone who is considering using teeth-whitening products to consult their dentist first.
"We continue to work with local Trading Standards and online platforms to identify unsafe products and ensure these are removed from sale - we expect the platforms to act swiftly when notified to keep people safe."