Parents of babies with milk and soya allergies could face charges of up to £112 per week to feed their children in London, it has been claimed.
Croydon NHS managers have decided to withdraw NHS prescriptions for formula feed to help reduce budget deficits.
Croydon Clinical Commissioning Group (CCG) said the plan could save the NHS thousands of pounds per year.
Mother Sarah Rose however said it was "like taking milk from a baby" as her seven-month-old son Sam depended on it.
Richmond Clinical Commissioning Group is consulting about withdrawing formula prescriptions too and the consultation runs until 3 February.
- Formula cannot be advertised for infants younger than six months in the UK
- Women in the UK are advised by the NHS to exclusively feed breast milk for the first six months, if possible
- The World Health Organisation has described infant formula as a specialised food that is vitally important for those babies who cannot be breastfed
It said: "It is proposed that GPs no longer provide soya-based infant formula milk, thickened infant formulas or formulas for lactose intolerance on prescription as these are now widely available to buy from community pharmacies and supermarkets at a similar cost to standard infant formula."
People who might be affected include those with cow's milk protein allergy and gastro-oesophageal reflux disease, it warned, but it calculated this would provide a saving of £386,000 per year.
Standard formula milk costs about £10 for a week's supply and is available in supermarkets and chemists.
Miss Rose, a 33-year-old primary school support assistant and mother-of-two, said Sam had a prescription for the specialist formula that would otherwise cost £40 online for one tin that would feed him for about two and half days.
She said: "The tins are about half the size of normal milk formula so it works out at eight times the price."
"We haven't got a lot of money. This is not me being fussy, but it's like taking milk from a baby", she said.
"We went to A&E twice before he [Sam] was diagnosed with this allergy at about four months. He was screaming in pain all the time. He arched his back and started refusing milk. Now he's a normal baby. I don't ever want to go back to that again."
An online petition calling on the CCG to reverse its decision has attracted more than 6,000 signatures and charities, such as Allergy UK and Anaphylaxis Campaign, expressed their disappointment and urged a rethink.
Dr Tony Brzezicki, Clinical Chair of NHS Croydon CCG said: "We share the public's concerns and we will do what we can to reduce the impact on the most vulnerable in our communities and to make sure funding is there for those with the greatest clinical need."
He said the prescriptions would cease in the coming months and families would be given notice of the plans before the change took effect.
"These are very difficult decisions but we need to focus our limited resources where we can have the biggest impact on people's health and well-being," he added.