Oxfordshire MP Boris Johnson has come under fire from a woman who lost her son in a road accident after he described a new law on child booster seats as "crack-brained."

The Henley MP, who was criticised last month for allowing his two sons to sit together in the front seat of a sports car, condemned the Government for forcing families to install safety seats for children aged up to 12.

Since Monday, children under 12 who are less than 135cm (4ft 5in) tall have had to use a child restraint in cars, vans and lorries.

The Tory frontbencher, writing in a national newspaper, said "hard-pressed British families" were being made to fork out up to £30 for "ludicrous plastic banquettes". He added: "If people decide that they are not going to comply with this crack-brained law, and they are not going to buy a banquette booster-seat for an 11-year-old, then they will have my complete sympathy."

But yesterday, road safety campaigner Jane Barber, whose 13-year-old son Josh Bartlett died in a crash on Oxford's Eastern Bypass last year, was critical of the MP's views.

She told the Oxford Mail: "I don't want to get into personal comments with Boris Johnson. But if he looked into the statistics, he should realise that these booster seats might one day just save the life of somebody from his family."

Her son was bring driven by the mother of a friend and only one of the seven boys in the car was wearing a seatbelt when the crash happened. Mrs Barber has been working with Oxfordshire County Council to raise awareness of child seatbelt safety.