Stephen Hawking, the British mathematician and physicist famed for his work on black holes, has been rushed to a hospital and is seriously ill, Cambridge University said.
Hawking has been fighting a chest infection for several weeks and was being treated at Addenbrooke's Hospital in Cambridge, the university city northeast of London, the university said.
"Professor Hawking is very ill," said Gregory Hayman, the university's head of communications. "He is undergoing tests. He has been unwell for a couple of weeks."
Later in the afternoon, Hayman said Hawking was "now comfortable but will be kept in hospital overnight."
The illness had caused Hawking to cancel an appearance at Arizona State University on April 6.
Hawking, 67, gained renown for his work on black holes, and has remained active despite being diagnosed at 21 with ALS, (amyotrophic lateral sclerosis), an incurable degenerative disorder also known as Lou Gehrig's disease.
For some years, Hawking has been almost entirely paralysed, and he communicates through an electronic voice synthesizer activated by his fingers.
Hawking was involved in the search for the great goal of physics - a "unified theory" - which would resolve contradictions between Albert Einstein's General Theory of Relativity, which describes the laws of gravity that govern the motion of large objects like planets, and the Theory of Quantum Mechanics, which deals with the world of subatomic particles.
"A complete, consistent unified theory is only the first step: our goal is a complete understanding of the events around us, and of our own existence," he wrote in his best-selling book, "A Brief History of Time," published in 1988.
In a more accessible sequel "The Universe in a Nutshell," published in 2001, Hawking ventured into concepts like supergravity, naked singularities and the possibility of a universe with 11 dimensions.
He announced last year that he would step down from his post as Lucasian Professor of Mathematics, a title once held by the great 18th-century physicist Isaac Newton. However, the university said Hawking intended to continue working as Emeritus Lucasian Professor of Mathematics.
"Professor Hawking is a remarkable colleague. We all hope he will be amongst us again soon," said Peter Haynes, head of the university's Department of Applied Mathematics and Theoretical Physics.
Brian Dickie, director of research at the Motor Neurone Disease Association, said only 5 percent of people diagnosed with ALS survive for 10 years or longer.