A growing market in the online sale of human breast milk is creating a serious health risk, doctors have warned.

British experts are concerned that the booming craze for breast milk - which is bought by adult fitness fanatics or those who have chronic diseases - could spread life-threatening disease.

An increasing number of adults are drinking breast milk, convinced that the health benefits it provides to infants could boost their fitness and immune systems.

Internet forums match buyers with breast-feeding mothers who have excess milk to sell.

Fetching £50 a pint or more, the milk is offered either fresh or frozen, and is often available by post.

UK doctors warn that the unlicensed breast milk market is posing a growing danger to health.

Writing in the Journal of the Royal Society of Medicine, they call for regulators to make it clear buying milk online is not safe.

‘The latest supplement – touted as completely natural, free-from and a ‘super food’ – human breast milk has emerged as a recent craze among adults,’ they said.

‘Online forums are replete with posts boasting about the immune, recovery, nutritional and muscle building benefits of human milk.

‘For those seeking a competitive edge, this milk is supposed to deliver significant returns.

‘A “clean” super food, it is purported to lead to ‘gains’ in the gym, to help with erectile dysfunction, to be more digestible and to contain positive immune building properties.’

But the experts, led by Dr Sarah Steele of Queen Mary University of London, said there is no evidence to back the claims.

Nutritionally, there is less protein in breast milk than cow’s milk, and clinical trials do not back the claim that adults can enjoy the health benefits experienced by brestfed babies.

More seriously, the doctors say human milk can be hazardous - spreading viruses such as hepatitis B and C, HIV and syphilis.

Because the milk is not pasteurised, it is also full of germs.

‘Research into breast milk bought online identified the presence of detectable bacteria in 93 per cent of samples,’ the authors wrote.

Dr Steele said: ‘While many online mums claim they have been tested for viruses during pregnancy, many do not realise that serological screening needs to be undertaken regularly.

‘Sexual and other activities in the [post-pregnancy] period may expose the woman expressing to viruses that they may unwittingly pass on to consumers of the milk.’

Many women who cannot breast feed their babies are eligible to get other mothers’ milk from regulated milk banks.

Dr Steele said that while milk banks are safe to use - with donors carefully screened - adults are not eligible to use them, so instead turn to the internet.

‘Buying online potentially exposes the consumer to bacteria, viruses and contaminants that render this not a “clean super food” for performance nutrition or supplementation.’