British scientist Stephen Hawking has been forced to miss a scientific debate to mark his 70th birthday but sent an upbeat message saying he was living at a "glorious time".

As scientists and media gathered at Cambridge University on Sunday for a symposium to mark Hawking's birthday, vice-chancellor Professor Sir Leszek Borysiewic said: "Stephen has been unwell and was only discharged from hospital on Friday."

But in a pre-recorded message, the physicist urged participants to focus on his glittering career and the future of science rather than his struggles against illness.

Advertisement: Story continues below "It has been a glorious time to be alive and doing research in theoretical physics," he said.

"We must also continue to go into space for the future of humanity," he argued. "I don't think we will survive another thousand years without escaping beyond our fragile planet."

Hawking, who was diagnosed with motor neurone disease aged 21, followed the special symposium on "the state of the universe" via webcast, the vice-chancellor said.

Despite spending most of his life in a wheelchair and being able to speak only through a computer, the theoretical physicist's quest for the secrets of the universe has made him arguably the most famous scientist in the world.

When Hawking was diagnosed with the debilitating condition aged 21, he was given only a few years to live, but has defied medical opinion by reaching his eighth decade.

Much of his work has centred on bringing together relativity (the nature of space and time) and quantum theory (how the smallest particles in the universe behave) to explain the creation of the universe and how it is governed.

In 1974, aged just 32, Hawking became one of the youngest fellows of Britain's prestigious Royal Society. Five years later he became Lucasian Professor of Mathematics at Cambridge, a post once held by Isaac Newton.

His fame moved beyond academia in 1988 with the publication of his book, A Brief History of Time, which explained the nature of the universe to non-scientists, and sold millions of copies worldwide.

Hawking's stardom was later cemented in cameos in Star Trek and The Simpsons, where he tells the rotund Homer Simpson that he likes his theory of a "doughnut-shaped universe", and may have to steal it.

As Hawking's age advances, he could be at risk of losing his famous computerised voice due to the gradual loss of muscle control, his personal assistant said ahead of his birthday.

"His speech has got slower and slower and on a bad day he can only manage about one word a minute," Judith Croasdell said in the Daily Telegraph.

"We think it may be because of the deterioration in his cheek muscle. We are looking to improve the situation and he needs to test out new technology."

Martin Rees, Britain's Astronomer Royal and a former president of the Royal Society, who first met Hawking when they were both research students, marvelled at his longevity.

He admitted that when they first met, "it was thought he might not live long enough to finish his PhD degree".

His survival made him a "medical marvel", Rees said, but stressed that it was his work that would prove his lasting legacy.

"His fame should not overshadow his scientific contributions because even though most scientists are not as famous as he is, he has undoubtedly done more than anyone else since Einstein to improve our knowledge of gravity."

Hawking has amyotrophic lateral sclerosis (ALS), a form of motor neurone disease that attacks the nerves controlling voluntary movement.

He admitted he felt "somewhat of a tragic character" after diagnosis, but he soon returned to work, securing a fellowship at Cambridge, and married Jane Wilde, with whom he had three children.

His birthday will also be marked by a new exhibition celebrating his achievements which opens at London's Science Museum on January 20.