A BBC Inside Out reporter posed as the father of a six-month-old baby and bought milk from mothers across the country.
The 12 purchases were then analysed by experts at Coventry University.
Microbiologists discovered a third of the samples contained E.coli, two contained Candida (thrush) and one contained the deadly bacteria pseudomonas aeruginosa.
This bug caused the deaths of four babies in a neonatal unit in Northern Ireland in 2012.
Websites including onlythebreast.co.uk have classifieds sections for people buying and selling breast milk.
There is also a section for men who wish to buy breast milk or who are interested in being breastfed.
Dr Sarah Steele from Queen Mary University of London told Inside Out that parents have “heard the message breast is best, which is absolutely the case but this is stuff bought off the internet”.
She added: “You don’t know the seller, you don’t know how they’ve been storing it, you don’t know what it contains and, more pertinently, they’re often doing this for profit and that poses the risk that they may tamper with it, water it down, be it with water, formula, cows milk or soya milk.”
She said the issue was a “real problem for infant health”, adding: “We don’t want to see a situation where a baby dies as a consequence.”
In June, researchers warned that buying breast milk online was a danger to health.
They said fetishists, bodybuilders and cancer patients all bought the milk through websites and social media groups.
The Queen Mary University of London team said the milk was unpasteurised and could carry dangerous germs.
Some 93% of breast milk sold online contains detectable levels of bacteria due to how it is expressed and stored, they warned.
Women who wish to donate surplus breast milk for free can use NHS “milk banks” to help premature babies.
Information on Milk Banks from the NHS at amazingbreastmilk.nhs.uk/support/milk-banks/.