Tommee Tippee has launched an investigation after receiving dozens of reports of mould building up in the internal pipes of machines which are used for preparing formula milk.
Concerned parents who own a Perfect Prep appliance have posted photos on the company's Facebook page of black residue they have discovered running through tubes hidden inside the machine.
The manufacturer says it believes the build-up is not mould, but harmless carbon residue which develops during the water filtration process.
But some mothers believe the machines may have been causing their babies to be unwell for prolonged periods of time - and insist the residue is mould.
Lindsay Platt-McHenry told Sky News she was "shocked and disgusted" after she unscrewed the back of her Tommee Tippee machine and found "black dirt" in the pipes.
Her 10-month-old son, Jack, has been into hospital a couple of times suffering from wheezing, sickness and diarrhoea - and she fears the appliance may have contributed to his illness.
The mother-of-two, from Stockport, said: "I was really upset that I had been feeding my baby from birth with this disgusting machine.
"You wouldn't ever see what's going on behind the machine unless you get a screwdriver and take eight screws out."
Ms Platt-McHenry said she does not believe the Perfect Prep machine should be on the shelves, and she has urged mothers to make sure the appliance's pipes are completely clean before use.
A significant number of Tommee Tippee customers on Facebook have said they have used these appliances for months without any residue building up.
Tommee Tippee is urging parents who have found similar build-ups in their appliances to contact them as soon as possible so it can arrange for the machines and filters to be returned for examination by an independent laboratory - but warned it is "impossible" to determine the exact cause until these inspections have taken place.
In a statement, the company added: "Our preliminary investigation has found two things. Firstly, the build-up we've seen so far we suspect is carbon residue and not mould. Carbon is a harmless, inert bi-product of the filtration process.
"Secondly, some parents have told us they've used ordinary water jug filters instead of the Tommee Tippee filters. Standard filters do not contain the ultra-filtration membrane needed to remove bacteria, plus are more likely to release carbon into the system.
"Although we can't be certain until we've received back and examined more machines, it is possible these factors are linked to what people are seeing. However, the investigation will continue until we've found answers."
Tommee Tippee said any parents with questions or concerns can call its helpline on 0500 97 98 99, or contact them on its Facebook page.