Renowned British scientist Stephen Hawking on Sunday had to miss a symposium to mark his 70th birthday because of ill health.Professor Sir Leszek Borysiewicz, Vice Chancellor of Cambridge University -- where the event is being held -- said Hawking was unwell, and had been discharged from hospital on Friday.

"Stephen has been unwell and was only discharged from hospital on Friday," Borysiewicz said, much to the disappointment of many scientists and reporters who had gathered to hear Hawking, one of the most brilliant theoretical physicists after Albert Einstein.

He said Hawking, who was diagnosed with motor neurone disease aged 21, would follow the symposium via webcast. His talk has been recorded and will be played in his absence.

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Hawking also had to miss all three days of a science conference held in his honour, the BBC reported. He has long defied and baffled medical experts who predicted he had just months to live in 1963 when he was diagnosed with Motor Neurone Disease, a deadly disease.

Only five per cent of people with the form of MND that he has -- a condition called amyotrophic lateral sclerosis or Lou Gehrig's disease -- survive for more than a decade after diagnosis.

The fact that Hawking, Britain's most celebrated physicist, has lived for nearly half a century with a condition that progressively attacks the nerves serving the muscles of the body has been described as remarkable.

"I have been lucky, that my condition has progressed more slowly than is often the case. But it shows that one need not lose hope," Hawking says.

"I am quite often asked: 'How do you feel about having ALS?' The answer is, not a lot. I try to lead as normal a life as possible, and not think about my condition, or regret the things it prevents me from doing, which are not that many," the BBC quoted him as saying.

Currently director of research at the department of applied mathematics and theoretical physics at the University of Cambridge, where he also founded the Centre for Theoretical Cosmology, Hawking previously (1979-2009) held the Lucasian Professorship of Mathematics at Cambridge, a post once held by Isaac Newton.

Although he has now stepped down from the Lucasian chair after a historic 30 years, he continues working at the University of Cambridge and recently published a new book -- The Grand Design. The grandfather-of-three continues to seek out new challenges.

Hawking recently experienced first-hand what space travel feels like by taking a zero-gravity flight in a specially modified plane.