The Queen Mary University of London team claimed the milk, which is sold through specialist websites and social media groups, was unpasteurised and could carry dangerous germs.
They said claims that the milk boosted the immune system were misleading and could be dangerous to cancer patients.
Popularly referred to as liquid gold, it is unclear how much of the product is being traded.
Dr Sarah Steele, from Barts and the London School of Medicine and Dentistry speaking with BBC said the market had "shown consistent growth around the world".
According to the report, 93% of breast milk sold online contains detectable levels of bacteria, as non-sterile equipment is used to express or store the milk posing threats like hepatitis B and C, HIV and syphilis.
The trade reportedly started to help women who were not producing enough breast milk themselves.
However, it is also bought for its supposed health benefits such as building muscle, by fetishists and even some "foodies".
Dr Steele particularly expressed worry over the milk being sold to cancer patients owing to the fact that the immune system is not functioning the same as in a healthy adult, thus posing great risk.