Thousands of women are sharing their own breast milk via social media groups in an effort to help others, a BBC investigation has discovered.
The Department of Health is now coming under pressure to issue more guidance to these mothers, who are acting outside of NHS supervision.
Some experts fear the unregulated practice could spread infection and viruses such as HIV and hepatitis.
But advocates argue mothers are making an informed choice.
When Bex Poole, from Wolverhampton, had difficulty breastfeeding her baby son Theo, she was anxious to find an alternative supply.
Theo was not putting on weight at a healthy rate but she does not drink cows' milk herself and was reluctant to supplement her son's diet with formula.
A friend suggested she look at a Facebook page called Human Milk for Human Babies UK, which facilitates breast milk exchanges between those mothers with surplus breast milk and others who need it.
Likes for the page have increased fivefold to almost 18,000 in the past five years.
"My milk wasn't increasing in any way," she said.
"I jumped straight on the page, no hesitation, and appealed for help."
'I had too much milk'
Shortly afterwards, she was contacted by Sarah McHugh, a new mother from Kidderminster, in Worcestershire.
She had struggled to breastfeed her daughter Harriet and had ended up expressing milk to feed her with.
"I ended up having too much milk," she said. "I'm on some Facebook groups for mums who express and breastfeeding mums so I put in a request saying I had some milk to donate."
The women's first meeting took place late at night and had an illicit feel to it.
"There was no other time we could do it," Ms Poole said. "It felt almost like a naughty transaction because her door is a little bit hidden behind some garages.
"Her little one was asleep, she was in her pyjamas ready to go to bed. I picked the milk up and came away but said thank you via text when I got home."
Ms McHugh said she felt happy something positive had come out of the difficulties she had experienced feeding Harriet.
"At the moment there is a very big drive to breastfeed.
"And some people that can't breastfeed or maybe can't make enough milk are feeling they have to explore [avenues such as online milk exchanges]."
Now the pair feel they have struck up a bond as a result of the exchange and Ms Poole's freezer is full of Ms McHugh's breast milk.
Is it safe?
Informal schemes such as this have, however, attracted some criticism from experts who question whether it is safe to feed strangers' milk to babies.
Ms Poole and Ms McHugh said the key to success was making sure you asked the right questions prior to exchange.
"I volunteered quite a lot of personal information," said Ms McHugh. "I said I was fit and well and that I wasn't a smoker and I also donate to the hospitals' milk bank, which I think reassured them."
"There's an unwritten trust among breastfeeding mums," Ms Poole said. "I don't believe a mum would share any milk if they've got problems."
The Facebook site offers guidance for anyone considering using it and urges people to discuss medications, alcohol or drug use. It suggests using a health care provider for further testing if worried and asking for copies of results.
Many countries already test for infectious diseases during routine prenatal/antenatal care, it says, and it suggests looking into home pasteurisation if worries persist.
However, Dr Gemma Holder, a consultant neonatologist at Birmingham Women's Hospital, is concerned mums who exchange milk without medical supervision might risk their babies' health.
She works at the hospital's milk bank, one of 16 official sites across the UK and Republic of Ireland, where donated breast milk is collected on a large scale and sent to sick and preterm babies in hospitals.
The donated milk is carefully vetted in line with NICE guidelines.
"When the milk comes in we first have to screen it for infection," Dr Holder said.
"Mothers who donate milk also have to have their bloods tested to ensure there's not a risk of blood-borne viruses - things like HIV, syphilis and hepatitis B - being transmitted to babies."
The milk is pasteurised before it is frozen, ready for use.
"Fresh donor milk has significant risk of potentially passing on infection, particularly if you don't know how it was handled," said Dr Holder.
"We know from just screening our milk there are bugs such as E. coli.
"We still get a couple of donors a month, for example, whose milk we aren't able to accept. This could be higher in the community, where none of these precautions are in place."
But Dr Sally Dowling, from University of the West of England, in Bristol, points out women have always shared milk with each other.
She said the World Health Organisation (WHO) supports feeding babies milk from another woman as an alternative to breastfeeding by the mother.
"The studies that have have taken place increasingly show that women make all sorts of judgements about risks for themselves," she said.
"They find out about the women whose milk they're acquiring - things like whether they washed hands when they expressed the milk, for example, or any infections or tests the women might have had done.
"Yes, there are some risks but on the whole women are going into this with their eyes open and finding out as much as they can."
'More guidance needed'
The Food Standards Agency says it does not recommend sharing donor breast milk for safety reasons.
"Parents wishing to donate, share or obtain breast milk should contact maternity or other medical services for advice," a spokesman said.
"Some NHS hospitals can provide donated breast milk for your baby."
However, experts have called for the UK government to do more.
Alison Thewliss, the SNP politician who chairs the all-party parliamentary group for infant feeding, believes the rest of the UK should follow the Scottish model.
One Milk Bank for Scotland is part of the health service and ensures breast milk can be collected from donors, processed and distributed using a well-developed network.
Ms Thewliss believes the Department of Health should take overall control of the breast milk donor services in England.
"At the moment, milk banks are often underfunded and running as a project of individual hospitals," she said.
"I would like to see the UK government work with the UK Association of Milk Banks to invest in services to allow those wishing to donate breast milk to be able to do so locally in a safe and regulated way, and for those requiring breast milk for their babies to be able to access it easily".