For parents the price of failing to ensure children are properly restrained on the back seat in cars is no longer just a safety issue - it could hit them in the pocket too.

From Monday, all children aged under 12 or under 4ft 5in tall must use a booster seat or cushion. Failure to do so would mean a £50 fixed penalty notice, or a fine up to £500 if the case goes to court.

Shops have reported unprecedented sales of booster seats, which can cost up to £70.

During the morning school run yesterday, opinion between parents dropping off their children was divided.

At West Kidlington Primary School, mother-of-three Debbie Long, 39, was carrying her seven-year-old son Lewis Harrison on a booster seat.

She said: "The booster seat is normally in my car anyway and I've always thought children should be in them even before the new law.

"The only other car my son goes in is my parents and I make sure they use a booster seat. I feel a lot safer if I know they are in a proper seat."

But mother-of-three Zena Leaman, 40, from North Oxford, believes many parents will be caught out.

She said: "I've got three children and when we go out in the family car using three booster seats it is a bit of a squash. People are going to have to get new cars.

"It's more and more Big Brother-type laws. If I was pulled over and ordered to pay a fine I would have quite a lot to say to the police - it's ridiculous."

Pauline Rushby, 36, of Kidlington, said: "It worries me that you still see kids rolling around in the back of the car and some parents don't even put them in seatbelts."

Father-of-five Geoff Brooker, 58, of Kidlington, said: "It is slightly over cautious and another piece of heath and safety overreaction."

Thames Valley Police roads policing department manager Malcolm Collis said studies of crash deaths in the Thames Valley revealed more than 80 rear seat passengers killed in crashes would have survived if they had been properly restrained.

He added: "Thames Valley welcome any changes which will further reduce the risk of death or injury on our roads, particularly to children who depend upon responsible adults for their safety."