Breast milk sold on the internet was generally not screened for diseases and contamination, adulterated with water and cow's milk, and tainted with high levels of bacteria, researchers from the Queen Mary University of London said in the latest British Medical Journal.

Lead author Sarah Steele said she was so alarmed by the team's early findings that she was prompted to publish an editorial calling for regulators to take urgent action and issue public warnings, fearing a baby could die.

She said mothers struggling to provide breast milk for their babies were increasingly turning to the internet where the breast milk trade was flourishing. The market was most rapidly expanding in the United States, where one website, OnlyTheBreast.com, has amassed 27,000 users.

"In the absence of warnings about the dangers of buying milk online, this option might seem healthy and beneficial -the better choice if one can't breast feed oneself," she wrote. "What mothers, and many healthcare workers, don't realise is that this market is dangerous, putting infant health at risk."

She said online breast milk was cheaper than traditional milk banks, which could charge up to $US4 an ounce. But the trade-offs were risky.

Black market breast milk vendors did not have to bear the cost of routine pasteurisation or testing for disease or contamination, her research showed. These cost-saving measures led to higher risks of disease transmission, contamination and tampering.

"Unlike donors at licensed milk banks online sellers are not required to undergo any serological screening, meaning that diseases such as hepatitis B and C, HIV, human T cell virus, and syphilis may not be detected," she said.

But Lisa Amir, a breastfeeding expert at La Trobe University, said Dr Steele was "over-exaggerating" the risks associated with "not a very big industry".

She said instant formula had risks as well, including the growth of bacteria.

"She's correct to say if someone buys milk off the internet, they don't know where it's coming from," Dr Amir said. "But I think she's over-emphasised it, a kind of scare-mongering."

A separate study showed 5 per cent of samples from licensed milk banks tested positive for cytomegalovirus, compared with 21 per cent of samples bought online.

Further, 90 per cent of samples bought online showed higher overall bacterial growth.

Another study found a quarter of 102 samples bought online arrived in severely damaged packaging and no longer frozen, which could lead to rapid bacterial growth and contamination.

And in a third study referenced by Dr Steele, researchers detected traces of chemical Bp and illegal drugs in milk purchased online. Some of the samples were mixed with water and cow's milk to boost volume.

The risk of disease transmission via tainted milk should also be a concern for bodybuilders, who consider it a superfood, and cancer sufferers, who mistakenly believe it has health benefits.

The Australian Breastfeeding Association said for mothers who did not have enough breast milk, another woman's milk was the next best alternative. But there were risks.

"The association strongly encourages mothers to ensure that they are well informed of the potential risks and benefits of donated human milk, methods available for minimising risks, and to make decisions based on their own individual circumstances," it said.

Since 2006, when Perron Rotary Express Milk Bank was set up in Perth, four banks have opened, including one at the Royal Prince Alfred Hospital's neonatal intensive care unit.

Facebook groups such as Eats on Feets or Human Milk 4 Human Babies allow Australian mothers to trade milk, but take no responsibility for outcomes and do not screen donors or provide absolute guidelines.

The World Health Organisation says: "The choice of the best alternative – expressed breast milk from an infant's own mother, breast milk from a healthy wet-nurse or a human-milk bank, or a breast-milk substitute fed with a cup, which is a safer method than a feeding bottle and teat – depends on individual circumstances."