Tommee Tippee has launched an investigation after parents complained of finding "mould" in the internal pipes of its formula feed machines.
The baby product company said it was concerned by reports of a "build-up" in the tubes of its Perfect Prep machines.
Parents have taken to social media to complain of feeling "angry" and "let down".
Tommee Tippee said initial investigations suggest the build-up was carbon residue which is "harmless".
The Perfect Prep machine is used to make a formula feed by controlling the quality, volume and temperature of the water being used.
The company posted a statement on its Facebook page saying it had received comments from parents complaining the internal pipes of the machines were not clean.
It urged families who had found a similar build-up to contact the company to have the machine returned to them for examination.
More than 3,600 comments have since been left on the Tommee Tippee page, with several parents saying they had also discovered unclean pipes.
Gillian Clelland, from Glasgow, said the machine she uses to feed her son Hayden was "full of black mould in the pipe".
She said: "The worrying thing is I don't know how long the mould has been there and if it has affected Hayden in any way."
Allana Simpson wrote: "I've just opened mine up to check after seeing this and there is black mould in the pipe. My poor wee boy. This won't be getting used. Horrified."
Holly Dack-Hughes said she had always done a "full clean" and changed the filter on her machine "every month" but opened it to find it was "absolutely disgusting".
"No wonder my daughter was having an upset tummy," she said.
'Good as new'
However other parents took to the page to say they had checked their machines and found the pipes were clean.
Astyn Hutchinson said the pipes of her machine - which she has she has had for 18 months and uses for every feed - were "completely clear".
She added: "I love this machine and has been a Godsend especially for those night feeds in the early stages."
Sarah Stephens said she bought hers in 2015 and had used it continuously for a year and found "all pipes clear and clean and looks as good as new" when she checked.
Tommee Tippee said it was "concerned" by the complaints made and has started an investigation.
It said it would be impossible to determine the exact cause until it had examined the machines sent back by parents.
A spokesman 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."