Experts have sounded the alarm over a growing trend for selling human breast milk online, warning that it can contain harmful bacteria, drugs and viruses including HIV.

The concerns prompted eBay to remove listings from its platform, saying it had updated its automatic filters to stop breast milk being sold on the site in future.

Dr Sarah Steele, a public health researcher at the University of Cambridge said that while human breast milk had been sold for some time on online platforms, it had normally been listed with some information about the potential risks.

But in recent weeks – after reports on social media of women earning as much as £10,000 from selling their milk to bodybuilders – she had noticed a marked rise in the number of adverts.

“We need to remember it’s a body fluid. Recently, I have observed, and what certainly ramped up very rapidly in the last week, is the rise of listings on auction sites and online social marketplaces with buy-it-now prices, minimal information about the seller, and no referencing to any safety advice or information whatsoever,” she said, adding there also appears to be an increase in listings offering to post the milk rather than courier it.

“What’s new is this idea of remote sale from a total stranger you will never meet and therefore aren’t screening or building trust with at all. You have no idea who or what is at the other end of the transaction. It’s very unsafe,” she said.

“Most of all shipping via Royal Mail is not temperature controlled and that’s when bacteria multiply. This should be obvious to people – you don’t send your mate milk via Royal Mail when they run out. Online grocery sites have had to implement a host of food safety protocols.”

One advert on eBay seen by the Guardian on Thursday that was apparently posted by a 36-year-old in the UK, charged £15 for 150ml of breast milk.

“My baby was born on 10 January and I have an oversupply of excess human breast milk for sale,” the seller wrote. “Milk will be posted frozen in insulated packaging with a frozen gel pack.”

After being alerted to the advert by the Guardian, an eBay spokesperson said: “The sale of human breast milk is prohibited on eBay. We have removed listings of this item and updated our automatic block filters to prevent it being listed in future.”

There are a number of milk banks in the UK, which provide free human breast milk to babies who need it. However these typically carry out lifestyle checks on donors for factors such as smoking or drug use, and screening of donors to check for infections including HIV, Hepatitis B and syphilis. The breast milk itself is also screened and pasteurised for safety.

Steele said one risk from unscreened milk is cytomegalovirus or CMV. While the virus is generally harmless it can cause serious health problems in unborn babies and infants and adults with weakened immune systems.

“While people assume the buyers are parents of newborns, actually quite a few people looking for milk online are adults, some who are looking for an alternative therapy when they have cancer or other conditions because they believe it will either offer an immune boost, or be a source of nutrition that they tolerate more,” said Steele.

“A lot of the promises people read about in online forums about the miracles of breast milk for adults don’t hold up to science, and the risks of buying this online outweigh any purported benefits,” she added.

Steele added research has previously found that human breast milk from online sellers can be rich in gram-negative bacteria, potentially linked to poor hygiene practices, which can be harmful.

Another risk, said Steele, is that human breast milk sold online may be diluted with cow’s milk or other fluid, while the storage bags used may mean the milk could contain other substances, such as the industrial chemical BPA.

• This article was amended on 21 February 2022 to remove a reference to cytomegalovirus (CMV) often being found in households with cats. Dr Sarah Steele has found milk samples that test positive for CMV and toxoplasmosis, the latter of which is a common infection that can be caught from the faeces of infected cats. While CMV doesn’t come from animals directly, it is commonly found in a test for detecting infections in pregnant women, which includes tests for both CMV and Toxoplasma gondii.