A mother from Honiton in Devon says her daughter can unbuckle herself from her car seat which is putting her life at risk.
Mel Fordham bought the product for £250 in Taunton.
She says the manufacturer responded to her complaint by saying she must have a 'clever child'.
Mel says she now has to have someone travelling in the back of the car with daughter Everly or have her forward facing to keep an eye on her.
She says she's unable to make the seat safer herself.
The manufacturer of the car seat, Joie, says car seat makers must follow strict guidelines when making car seats.
Mel says she hopes what's happened with Everly can protect other children.
Information on how to prevent your child from undoing their harness can be found on the RoSPA website.