McDonald's is a sponsor for the London Olympics -- and a British doctors' group says that's sending the wrong message in a country with ballooning obesity.

Big Macs, fries and milkshakes will be part of McDonald's exclusively branded menu at the Olympics and the fast-food giant will soon be opening its largest franchise in the world, a two-story cathedral-like restaurant that seats 1,500 customers, at London's Olympic Park. 

McDonald's will be the only restauranteur allowed to sell brand-name food at the Games and there will also be a separate McDonald's within the Athletes Village -- in addition to three others at the Olympic Park.

Alongside McDonald's, Coca-Cola has the exclusive right to sell non-alcoholic drinks at Olympic venues. Heineken has been named the Games' official beer.

"It's very sad that an event that celebrates the very best of athletic achievements should be sponsored by companies contributing to the obesity problem and unhealthy habits," said Terence Stephenson, a spokesman for the Academy of Royal Medical Colleges.