British theoretical physicist Stephen Hawking has identified what he believes is “one of the most serious public health problems of the 21st century” – our sedentary lifestyle.

The 74-year-old, who was recently hospitalised in Rome due to health concerns, warned “millions of lives are in danger” due to “complications from over being overweight or obese”.

“We eat too much and move too little,” Hawking said in a video for not-for-profit Swedish health organisation GEN-PEP, release on Saturday.

“Fortunately the solution is simple, more physical activity and change in diet. It’s not rocket science.”

Hawking, who is regularly regarded as the smartest man in the world, also questioned how the problem had become so dire.

“For what’s worth, how being sedentary has become a major health problem is beyond my understanding,” Hawking said.

Watch Hawking’s warning below

The author of A Brief History of Time, Hawking was diagnosed with ALS, a type of Motor Neurone Disease, shortly after his 21st birthday, leaving him wheelchair-bound.

Miraculously, Hawking has defied a diagnosis involving a drastically shortened life expectancy to continue travelling the world with a team of carers and staff, delivering sold-out speeches through a computer.

He was in Rome to give a talk at the Vatican on the origins of the universe and to meet with Pope Francis, despite being a self-described atheist, to discuss climate change.

Hawking also recently penned an opinion piece for The Guardian reacting to the election of US president-elect Donald Trump as well as Britain’s vote in favour of leaving the European Union.

“What matters now, far more than the choices made by these two electorates, is how the elites react,” Hawking wrote.

“Should we, in turn, reject these votes as outpourings of crude populism that fail to take account of the facts, and attempt to circumvent or circumscribe the choices that they represent? I would argue that this would be a terrible mistake.”