Professor Stephen Hawking, one of the world's most famous scientists, is "very ill" in hospital, Cambridge University said today.
Professor Hawking, who works at the university, was undergoing tests at Addenbrooke's Hospital, Cambridge.
A university spokesman said the 67-year-old physicist, who is best known for his book A Brief History of Time, was taken to Addenbrooke's by ambulance.
"Professor Hawking is very ill," he added.
Professor Hawking suffers from motor neurone disease and is wheelchair bound. He speaks with the help of a voice synthesiser.
He developed symptoms of the disease while studying in the 1960s and is one of the world's longest surviving sufferers.
He has worked at Cambridge's Department of Applied Mathematics and Theoretical Physics for more than 30 years and since 1979 has been the University's Lucasian Professor of Mathematics.
Professor Hawking was awarded a CBE in 1982, became a Companion of Honour in 1989 and is a Fellow of the Royal Society.
He lives in Cambridge and has three children and one grandchild.
Professor Hawking was born in Oxford but his family moved to St Albans, Hertfordshire, when he was eight.
He studied at St Albans School before reading physics at University College Oxford then moving to Cambridge to carry out research in cosmology.
One of Professor Hawking's last public appearances was last September when he unveiled a £1 million clock erected at Corpus Christi College, Cambridge.
"Professor Hawking is very ill and has today been taken by ambulance to Addenbrooke's Hospital, Cambridge," said the University spokesman.
"He is undergoing tests. He has been unwell for a couple of weeks."
Professor Peter Haynes, Head of the University's Department of Applied Mathematics and Theoretical Physics, added: "Professor Hawking is a remarkable colleague. We all hope he will be amongst us again soon."