Professor Stephen Hawking was 'very ill' last night after suffering a serious respiratory infection.
The world-renowned scientist and writer, who is paralysed with motor neurone disease, was taken to hospital by ambulance for 'urgent tests' after his condition worsened.
Doctors said the 67-year-old author of the world's most popular science book, A Brief History of Time, was due to spend the night in hospital.
At his side were his daughter Lucy, 38, and his first wife Jane, 65, who is now married to choirmaster Jonathan Hellyer Jones.
He said: 'It is a very distressing time. We are waiting for news and we only learnt that something was wrong this morning.'
Professor Hawking was diagnosed at 21 with the muscle-wasting disease, which has gradually robbed him of his voice and movement in his limbs.
He speaks through a distinctive voice synthesiser and uses a motorised wheelchair.
The scientist was told he could expect to live for two years but has defied the prediction to become Britain's oldest-known survivor of motor neurone disease.
Professor Hawking returned from a work trip to America on Saturday after ill-health forced him to cancel appearances.
He was taken to Addenbrooke's hospital, Cambridge, yesterday lunchtime for tests which a close colleague said were 'far more serious than routine'.
A spokesman for Cambridge University, where he is Lucasian Professor of Mathematics, said Professor Hawking had been unwell for a couple of weeks.
'He is very ill,' said the spokesman. 'He had a respiratory infection in the States and had to cancel appearances.
'We are all very concerned for his health, as are his family, immediate colleagues and students.'
Professor Hawking, who also has two sons Tim, 30, and Robert, 42, and a grandchild, is due to retire from his post in October but planned to continue to work at the university.
One of his last public appearances was last September, when he unveiled a £1million clock at Corpus Christi College, Cambridge.
Professor Peter Haynes said: 'He is a remarkable colleague. We all hope he will be among us again soon.'
Professor Hawking's complicated family life has also attracted attention. In 1990, on his silver wedding anniversary, he left his wife Jane to move in with Elaine Mason, a nurse who had cared for him. They were married in 1995.
The bitter split astonished colleagues and friends, and his new wife was later accused by his family of bullying him.
In 2000, detectives launched an inquiry after Professor Hawking made a number of visits to Addenbrooke's suffering cuts and bruises. Another inquiry was opened in 2003.
Professor Hawking denied that his second wife was to blame. The couple divorced three years ago.
In 1988 A Brief History of Time, which sold more than nine million copies and made him a household name, was published.
His fame led to guest roles in The Simpsons, Red Dwarf and Star Trek: The Next Generation.
One of his greatest achievements was the discovery of 'Hawking radiation' - the way a black hole leaks energy and fades to nothing. Mail Online reported.