Some 500 children a year are seriously injured travelling in cars

Mother & Baby magazine also found 11% of parents had not bought seats, while a third of parents take children out of seats to comfort them while travelling.

"Even a collision at 5mph can kill an unrestrained child and a badly fitted car seat is as useless as no car seat," magazine editor Elena Dalrymple said.

Mother & Baby conducted a nationwide survey of 2,000 parents.

'Parents unsure'

More than half of parents allow their children to travel in other people's cars, particularly those of grandparents, without a car seat.

A quarter of parents admitted they sometimes fasten one seat belt around themselves and their child - an arrangement that would see the youngster crushed in the event of an accident.

Seats will often look as though they are fitted properly, but when an expert looks they will find fault The Royal Society for the Prevention of Accidents

The Royal Society for the Prevention of Accidents (Rospa) was not surprised by the poll's findings.

"It is a well known problem," said Rospa's Roger Vincent.

"It is regularly publicised but sadly parents are still unsure of what they need to do, or just don't bother."

He also pointed out the need for constant publicity to reach new parents.

Mr Vincent said it was essential for parents to make sure their seat is the right size for their child and suitable for their make of car.

Mother & Baby found 64% of parents did not know if their seat was designed to fit their particular car.

'Second-hand' warning

Parents should insist on a demonstration of how to fit the seat when they buy it and always follow instructions, Mr Vincent said.

"Seats will often look as though they are fitted properly, but when an expert looks they will find fault," he said.

He also warned against buying second-hand seats, where parents would not know its accident history and be unlikely to have instructions.

Some 12% of parents admitted using second-hand seats in the magazine survey.

Every year in the UK, around 40 children are killed and more than 9,000 injured while travelling in cars.