Dame Judi Dench joined calls last night for a ban on smoking in cars, citing the dangers to the health of youngsters.
"Children are still being exposed to passive smoke, which is extremely harmful to developing lungs," said the actress.
Dame Judi, whose husband Michael Williams died of lung cancer in 2001, is vice president of the British Lung Foundation, which is calling for a ban.
She added: "I am happy to offer my support ... I encourage anyone who wants the Government to make children's lung health a priority to sign up to the petition."
Pressure is mounting on the Government to extend the smoking ban to cars. Such a move would prevent some of the 22,000 new cases each year of asthma caused by passive smoking, the charity claims.
Research published in The Lancet last month showed 40 per cent of children were exposed to passive smoke each year, accounting for 165,000 deaths worldwide.
Dame Judi is one of 12,000 people to have signed the petition calling for a ban on smoking in cars carrying children. The charity's aim is to collect 50,000 signatures. Deborah Arnott, the chief executive of Action on Smoking and Health, called on the Govenment to continue campaigns discouraging parents from smoking in front of their children. She said: "While we can't pass legislation to prohibit smoking in the home, smoking in cars can and should be prohibited by law."
Simon Clark, the director of the smokers' group Forest, conceded: "It's reasonable to encourage people not to light up in a small confined space if children are present," but he dismissed calls for a ban as "unnecessarily heavy-handed". He added: "It's a small step to far more illiberal measures like banning smoking in all private vehicles or, worse, banning smoking in the home. Enough is enough."
A ban cannot happen quickly enough for Lynda Mitchell. The 53-year-old, from Bristol, has never smoked but is dying of chronic obstructive pulmonary disease. "I found being exposed to passive smoke in the car the worst because there was no escape. I want parents to know that what happened to me could happen to their children if they continue to smoke near them."
Smoking in vehicles carrying children is illegal in parts of Australia, Canada, the United States (including California), as well as Cyprus. Mauritius is the only country to have banned smoking in cars outright.