A tightening of the rules for food and drink advertising to children, expected to be unveiled by Ofcom in the next 10 days, could have a serious impact on sponsorship of the London 2012 Olympics.

Coca-Cola and McDonald's, which are Olympic Partners, are most likely to be affected. They hold multimillion-pound long-term contracts with the International Olympic Committee, giving them worldwide marketing rights of association with the Olympic movement.

Any time restrictions on advertising to children, as proposed in the Ofcom consultation, would affect their opportunities to advertise their products at certain times. Although the games are being broadcast on the BBC, highlights will be shown on commercial stations and sponsors will want to make the most of their investment.

Ofcom's ruling will also form the basis for a strengthening of radio and non-broadcast rules, potentially affecting the brands' abilities to advertise on pack, in-store and by direct mail.

John Whittingdale, chair of the Culture, Media and Sport Select Committee, has tabled parliamentary questions on the issue but has not yet had a response. He is to bring up the issue with Culture, Media and Sport minister Tessa Jowell at a select committee hearing on 21 November. "We at least need to be aware of the implications of restricting advertising," he said.

Sponsorship contributes over 40% of Olympic marketing revenue, with companies entitled to marketing rights, category exclusivity and use of Olympic images.

The situation could be worse if Ofcom went for a pre-9pm ban, said Andrew Brown, chairman of the Committee of Advertising Practice. "Communication of the Olympic sponsorship could then be problematic."

Christopher Graham, director general of the ASA, said: "We are demonising companies' advertising, then picking their pockets for the Olympics."

A Coca-Cola spokesman said it was working with government and Ofcom on self-regulation.