The group is calling upon the British government to restrict advertising by McDonald's, Coca-Cola and Heineken during the Olympic Games, which are being held in London from July 27 to Aug.12.

But that's unlikely to happen. London Olympic organizers have defended their decision to accept McDonald's sponsorship as a business deal.

"Sponsors provide a huge amount of the funding required to stage the games," said a London 2012 spokesman in a statement. "Without our partners such as McDonald's, the games simply wouldn't happen."

About one-quarter of Britons are obese and experts estimate that could jump to half by 2030. Obesity and related health ailments cost the U.K. health system about 4 billion pounds ($6.5 billion) every year.

"These brands are using the Olympics to be associated with medals and svelte, fit athletes," he said. "They don't want us to think of fat, unhealthy people when we think of their products."