Most cosmetic items sold by third-party sellers on Amazon, eBay, TikTok and Vinted are fake and could be a health hazard, a study suggests.

Researchers described their findings as 'alarming' and warn people 'enticed' by discounts on counterfeit designer goods risk being harmed by 'toxic' ingredients.

They purchased 34 makeup and skin products from sellers that have collectively sold thousands of these items.

This includes products advertised as being from eight well-known brands including Charlotte Tilbury, La Roche Posay, Maybelline, MAC and The Ordinary.

But investigators from consumer champion Which? concluded 23 of the 34 items (67 per cent) are likely to be counterfeit.

In each case, they conducted visual checks on the packaging and contents, and compared them with the genuine products purchased from trusted retailers.

Four out of 11 products ordered on Amazon were deemed to be fake, as were eight out of 11 products purchased on eBay, five out of six products ordered from TikTok shop, and six out of six items bought on Vinted.

Doctors say the consequences of using counterfeit products can be 'grave'.

Fake cosmetics, perfumes and other beauty and hair products have previously been found to contain animal faeces and urine used as a stabiliser, as well as cancer-causing and toxic ingredients such as arsenic, lead and mercury.

Consultant Dermatologist Dr Aamna Adel said the lack of regulation or oversight in the production of fake goods means they may be incorrectly formulated even if they contain genuine active ingredients, such as glycolic acid or retinol.

This poses the risk of skin irritation, skin infections or even chemical burns, she added.

In contrast, legitimate cosmetics undergo safety checks and are manufactured under controlled conditions.

Which? said many of the dupes were difficult to spot and only became apparent when directly compared with the genuine samples.

A bottle of Charlotte Tilbury Airbrush Flawless Setting Spray looked the part from the outside, but on closer inspection researchers noted it had a cloying floral scent, distinctly different from the genuine sample.

Similarly, a MAC Macximal silky matte lipstick in shade Ruby Woo purchased from Vinted for £15 - £10 less than the recommended retail price - arrived in packaging very similar to the genuine version, but when closely compared, the lipstick bullet itself was subtly different with a smudged logo that did not match up with the original.

A tube of La Roche Posay Effaclar Serum purchased on Vinted smelt suspiciously like shampoo, while the sample bought from Vinted had completely different text on the side of the box.

When Which? researchers purchased discounted tubes of CeraVe Resurfacing Retinol Serum from Amazon, eBay and Vinted, all three came in different packaging to the genuine product and were missing the safety markings researchers would expect to see.

It was difficult to get any serum out of the bottles using the pumps, but when they did it was clearly different in consistency and colour.

The Ordinary product listings were some of the most popular researchers found.

The two listings on eBay had collectively sold more than 2,600 items and nearly 1,000 were sold on Tiktok Shop.

This was 'all the more surprising' as these were among some of the most affordable items in the investigation, with products all retailing at less than £10, researchers said.

Phil Lewis, Director General of The Anti-Counterfeiting Group, said even on cheaper fake products the profit margins are 'extremely high', meaning consumers should be on their guard even if discounts seem more realistic.

Which? is calling on the government to reform the enforcement system and advising shoppers to be vigilant when buying items from online marketplaces.

It said the safest way to buy cosmetics products online or in-store is to buy directly from the brand website, or one of their affiliated retailers or approved stockists

Rocio Concha, director of policy and advocacy at Which?, said: 'It's alarming that Which? was able to find such high volumes of what appeared to be potentially dangerous counterfeit cosmetics on popular online marketplaces, where teenagers in particular are likely to be enticed by what seem to be big discounts on popular brands.

'Consumers should be as confident in the products they buy online as the items that they purchase on the high street, but in reality they risk buying untested, unregulated and potentially toxic cosmetics.

'Trading Standards needs to take decisive action to prevent these products from making their way into people's homes, but because of limited resources and competing demands, counterfeit investigations are being deprioritised in many parts of the country.

'The government must ensure there are clear duties on online marketplaces to prevent the sale of counterfeit goods, and urgently commit to the reforms needed to ensure our consumer enforcement system is fit for the 21st century.'

The online retailers all said they prohibit the sale of counterfeit items and take action to prevent fakes being sold.

A spokesperson for L'Oréal Groupe, the parent group for CeraVe, La Roche Posay, L'Oreal and Maybelline said: 'L'Oréal Groupe takes the threat of counterfeit products very seriously.

'We are committed to the fight against counterfeiting to protect consumer health and safety, and the integrity of our brands,' it added.

'We actively collaborate with law enforcement agencies worldwide and with online platforms.'

Charlotte Tilbury and The Ordinary did not respond to the researchers' requests for comment.