Penny Powell, a mum from Montreal, Canada, posted photos to Facebook of different cups, showing her mouldy findings in the anti-spill guard valve that holds the mouthpiece together.
• READ MORE: Is your kid's drink bottle making them sick?
Translated from French, she wrote: "My friend wondered why his son was still sick so he broke open the anti-spill guard of his Tommee Tippee cup and discovered mould inside the anti-spill top that you can't see except if you break it open."
The original post has been shared over 100, 000 times.
Thousands of parents have also posted photos of their children's mould infested cups, some imploring fellow mums and dads to throw the product out.
Tommee Tippee representatives have responded to the online outrage, claiming to be "actively working on the issue."
"We learned, as a result of posts, that some people were not satisfied with the result of the sippy valves used in our sippy cups: We apologise for the inconvenience," they wrote on Facebook.
A spokesperson told BuzzFeed: "The results support that when used with recommended liquids (cold, light fluids including water and non-pulp juices) and cleaned in line with instructions, there are no problems. Difficulties have arisen though when liquids that are not recommended for use in the cups have been used, like thick formula milk, pulpy juice and warm liquids.
They also shared instructions for proper cleaning, adding, "We also recommend that cups are not left for long periods before being cleaned."
- nzherald.co.nz
