A recent survey showed that two-thirds of the 42 commercials broadcast during weekend cartoon programmes were advertising junk foods and snacks, said Dr Wittaya Kulsomboon of Chulalongkorn University's social pharmaceutical research unit.

He said advertising aimed at children was expensive, with nine minutes on air costing up to Bt900,000.

There are 155 million overweight children worldwide and up to 45 million are clinically obese, said dentist Krisada Reungareerat of ThaiHealth. Krisada said adverts and TV commercials were contributing to the problem.

A study by the UK Office of Communications found that children under eight were the easiest group to convince because they could not separate reality from advertising, especially if the messages came from cartoons or famous people. Two-thirds of children aged six and seven said they believed what the commercials conveyed.

The survey said the most influential adverts were those giving away toys, followed by those presented by famous actors.

The Consumer Protection Network, the Thai Health Promotion Foundation and parent networks want a law change to stop advertisers using actors, singers, cartoon characters, mascots and children under 14 in commercials.