The prime minister, a long-term supporter of light-touch ad regulation, took the industry by surprise in a speech warning rules would be made mandatory if a voluntary code failed to work by next year.
But his remarks confused the Advertising Association and it plans to ask Downing Street to clarify them.
Blair tried to play down fears Labour was creating a "nanny state". But he added: "I would say my thinking has changed more towards intervention and away from letting things happen. What I do think you will see more of is the Government intervening directly with the food and drinks industry where, for example, you have the sale of junk food to children and the irresponsible marketing of alcohol."
He said the government's successful anti-smoking campaign had been partly responsible for his rethink.
However, Blair's hard line has stunned the AA. Jeremy Preston, the director of the AA's Food Advertising Unit, asked: "What does he mean?"
He also hit back at the government for expecting a reduction in obesity and binge drinking too quickly. "Over the past two years, companies have done an enormous amount to help," he said.
What bewildered the industry most was Blair's warning about "mandatory" controls. Preston said: "TV advertising is under Ofcom's control -- the rules are already mandatory."
He added: "Is Blair really saying that unless obesity and drink problems have been shown to be reduced by next year, the voluntary code has not worked? It's impossible to expect that so quickly."
Privately, ministers say they want to avoid legislation but that a statutory approach is the "default position".
Pic: Crown Copyright
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