Researchers uncovered listings for the substances despite their potential side effects including increased blood pressure and heart rate. High doses of the drugs could lead to strokes, heart attacks and kidney damage.

Diet pills are often marketed to body-conscious people as weight loss supplements and workout enhancers. They often contain two substances – yohimbine and synephrine – that the Medicines and Healthcare products Regulatory Agency (MHRA) has said have “considerable potential to cause harm if used without medical supervision or advice.”

However, despite these warnings and the fact that yohimbine and synephrine are not meant to be routinely available for sale without medical supervision, Which? was able to purchase and have delivered a sample of three items each from eBay, Wish and AliExpress respectively.

Each online marketplace had at least a dozen products listed on their websites that contained yohimbine or synephrine at the time of the consumer champion’s investigation.

Of the nine products Which? purchased, two carried no health warnings or dosage information at all. One had the name of the ingredient – in this case yohimbine – clearly visible on the outer packaging, though it still made its way into the UK and to the address of the recipient.

In response to Which?’s investigation, eBay and AliExpress said they had removed the product listings concerned. Wish said it was in the process of removing them, though at the time of writing yohimbine and synephrine products were still available on the site.

Which? says that the measures to prevent listings appearing in the first place appear not to be not working.

Prices on the supplements varied considerably. Some items were priced from as little as £2.39 plus postage, though some capsules were priced at more than £80. They were available to be shipped from countries including India, the USA, Ukraine and Poland, with many offering free postage.

Yohimbine and synephrine generally could lead to symptoms such as agitation, aggression, nausea, and increased heart rate and blood pressure.

Both supplements are popular among some bodybuilders and gym goers but there have been a number of studies that have warned of potential side effects, while some consumers have reported unverified concerns about them on social media and forums.

Which?’s investigation raises serious concerns over the lack of checks and monitoring carried out by online marketplaces and highlights the need for them to have legal responsibility for unsafe products on their sites.

Earlier this month the consumer champion carried out an investigation into dangerous toys sold on the same online marketplaces.

Currently, responsibility sits with the third-party sellers on these sites but there doesn’t appear to be adequate enforcement action taken to stop potentially unsafe supplements and medicines being sold to UK consumers. This could leave people worryingly exposed to products that can cause harm.

Sue Davies, Which? head of consumer protection policy, said: “It is concerning that our investigation has revealed these slimming supplements containing potentially dangerous ingredients are readily available on online marketplaces. The limited regulation of these sites is not working – and that’s leaving people exposed to substances that can be harmful.

“Online marketplaces must be given greater legal responsibility for the safety of products sold on their sites, so that shoppers are far better protected. Regulators also need to be more proactive in policing potentially dangerous products that are offered for sale on these sites, which are becoming increasingly popular places to shop.”