Safety experts say the extra layers can be seriously dangerous as they potentially create a gap between a youngster's body and the seat belt.
This means that if they are in a crash, the harness is not close enough to the child's body to properly restrain them.
Parents are being urged to strap children in without their coats – and to make sure the harness is done up tightly enough that they can only just get two fingers between their little one and the straps.
Good Egg Car Safety has issued advice for keeping tots warm in the car – including dressing babies in thin layers or folding a blanket in half to tuck tightly around them.
The blanket should not come up any higher than arm pit level.
For older children, parents should remove coats and jackets then also tuck a blanket around them that they can remove themselves.
Jan James, CEO of Good Egg Safety said: "We are the leading child seat specialist in the UK and since January 2017, we've checked nearly 5,500 car seats for parents and carers at free community check events.
"We found that 65 per cent of child seats or children were either incorrectly fitted or the seats were incompatible with the car.
“We’ve been tirelessly campaigning for the safer transport of children since 2001 and have made safe over 33,500 child seats and the children within them during this time.
"It’s an ongoing struggle as parents can understandably get confused by the different seat legislation and are not always getting the safest advice when buying them."
More information can be found on goodeggcarsafety.com.
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