Half of all parents did not even know if their child seat was the right one for their car, the survey found.
As many as 55% of parents had used second-hand car seats which were "probably completely useless", said Mother & Baby Magazine, which conducted the survey.
Also, 90% of parents said they had problems fitting seats and fewer than 6% were offered fitting advice by the vendors.
The magazine identified four main car seat problems:
The survey also found that one in five parents allowed their baby or small child to travel in someone else's car without a car seat and one in three admitted fastening a seat belt around two small children or fastening a belt around themselves and a small child.