Which? has warned that these products often contain illegal levels of hydrogen peroxide, which can burn gums and cause permanent damage to teeth.

An investigation by the consumer champion uncovered strips, gel-filled syringes and pens with brush tips that were dangerously overloaded with one of the active ingredients used for teeth whitening.

Worryingly, 21 of the 36 teeth whiteners available on online marketplaces that were tested in the lab exceeded the legal amount of hydrogen peroxide permitted for home use.

Teeth-whitening products sold over the counter should legally only contain up 0.1% hydrogen peroxide, while the legal limit for application by professional dentists is 6%. However, many of the products Which? tested exceeded these levels by a significant margin.

In Which?’s tests the six most dangerous whiteners had more than 100 times too much hydrogen peroxide to be legally sold. Five of these products were purchased from AliExpress. The worst one, a ‘Teeth bleaching gel kit’ sold by Oral Orthodontic Materials store, contained 30.7% hydrogen peroxide.

Despite Crest Whitestrips not being officially sold in the UK, Which? found them being sold through a China-based Wish seller willing to ship to the UK. In lab tests, the strips were found to contain more than 132 times the legal amount of hydrogen peroxide. Procter & Gamble, owner of the Crest brand, told Which? that it doesn’t sell this product in the UK or Europe, as the law doesn’t permit its sale there.

Overall, of the eight whitening products Which? bought from AliExpress, seven had too much hydrogen peroxide to be legally sold in the UK. For Wish, four out of six breached legal limits. Five out of nine of the products tested from eBay broke the law on hydrogen peroxide levels – the worst one contained 7.43%. Five out of 13 of the products tested on Amazon Marketplace had illegal amounts of hydrogen peroxide – the worst offender was a pen containing 7.87% of the chemical.

All of the online marketplaces removed the unsafe products that went above legal limits from sale when they were flagged to them by Which?.  The consumer champion says consumers to seek help from a professional dentist if they are looking to get whiter teeth.

Which? believes the findings in its tooth whitening investigation raise further question marks over the checks and monitoring carried out by online marketplaces. It’s calling for the government to give online marketplaces legal responsibility for the safety of products sold on their sites.

Sue Davies, head of consumer protection policy at Which?, said: “It’s worrying that our tests have revealed so many of these products sold on online marketplaces – and often hyped on social media – are breaking legal limits for hydrogen peroxide and putting the health of users at risk.

“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.”

An AliExpress spokesperson 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.”

An Amazon spokesperson said: “We require all products offered in our store to comply with applicable laws and we monitor the products sold in our stores for product safety concerns. When appropriate, we remove a product from the store, reach out to sellers, manufacturers, and government agencies for additional information, or take other actions.”