IT looked like one big happy meal — Sir Elton John sang Happy Birthday and Colin Powell, former US secretary of state, raised a glass to the birthday boy.

Ronald McDonald, the American burger giant’s jaunty corporate clown, celebrated his 50th birthday last night at a black-tie gala dinner at the Arie Crown Theatre in Chicago.

Half a century ago this weekend, Ray Kroc, McDonald’s founder, opened the first McDonald’s restaurant in Des Plaines, just outside Chicago, and proudly placed his corporate mascot, Ronald, outside the front door.

But last night, as the champagne flutes clinked and waiters handed out mini Big Mac canapes, McDonald’s newly appointed chief executive, Jim Skinner, cast anxious glances around the ballroom on the banks of Lake Michigan.

Skinner has good reason to be concerned. He is about to take a giant corporate gamble. If it comes off, he will re-establish McDonald’s as the most successful food company in the world. If he fails, Ronald will not make it to his 60th birthday.

Skinner, a company veteran who started out peeling potatoes in a McDonald’s in Davenport, Iowa, in 1962 and got the top job last year after his predecessor, Charlie Bell, died of cancer, is declaring war — not on the anti-obesity campaigners or environmentalists who target McDonald’s, but on his own customers.

Skinner is exasperated that in spite of spending hundreds of millions of pounds introducing low-fat dishes from salads to fresh fruit and vegetables, McDonald’s is still blamed for the obesity epidemic. Privately, senior McDonald’s executives complain that while many customers say they will buy the new healthy meals, when they get to the counter they choose burger ’n’ fries. In both Britain and America, 90% of McDonald’s sales are hamburgers. Salads account for less than 10%.

In a bold — some say reckless — move, Skinner is trying to shift responsibility for obesity away from the fast-food firm and onto its own consumers. In an interview with The Sunday Times at the opening of McDonald’s biggest restaurant in Chicago last week, Skinner, 61, said his message to consumers was: we are doing our bit to tackle problems of diet and obesity, now you do your bit.

“It’s time to shift the focus to personal responsibility,” he said. “It’s not just about our products any more, it’s about what our customers do. I hope people don’t think we are preaching the message ‘eat less and, by the way, get up off your butt and get out there’, but we do want to remind them that leading a balanced, healthy life is important.”

In a series of new advertisements that will be screened in Britain and America during the summer, McDonald’s will poke fun at consumers for not taking enough exercise.

One of the advertisements features adults lazing in front of a television set, with the slogan “Maybe you should spend less time with your TV”. In another commercial, a child is pictured running through a field with a television set tied to his ankle. A third ad shows a boy struggling to drink the recommended eight glasses of water a day for children. In a celebrity spot, Serena and Venus Williams play tennis before going to McDonald’s for lunch. Each commercial ends with the slogan: “It’s what I eat and what I do and I’m lovin’ it.”

To back up its new ad campaign, McDonald’s is releasing figures that show that in most western countries calorie intake among teenagers and young adults has risen by 1% in the past 20 years while physical education has declined by more than 13% in the same period. “That figure shows you how important exercise — not just food — is to the obesity debate,” said Skinner.

To encourage customers to take more exercise, McDonald’s will give them daily recommendations for physical exercise, alongside recommendations for calories, fat and sugar. The firm will also issue pedometers to encourage consumers to walk more. Computer programs for children will remind youngsters to take an activity break every 20 minutes.