Professor Stephen Hawking has joined the Swedish royals in a campaign addressing serious obesity and physical inactivity.

In an ad titled "Pep talk with Stephen Hawking" the cosmologist and physicist addresses "the most serious public health problems of the 21st century."

"Today too many people die from complications related to overweight and obesity. We eat too much and move too little," Professor Hawking says in the ad shot for Swedish non-profit GEN-PEP.

The solution, he says, "is not rocket science," and he recommends that people eat less and take up more physical activity.