A spokesman for Cambridge University said Prof Hawking was brought to Addenbrooke's Hospital today and is undergoing tests.
The 67-year-old suffers from the type of motor neuron disease known in the US as Lou Gehrig's disease, is confined to a wheelchair and speaks through a voice synthesiser.
Probably one of the world's best-known theoretical physicists, Dr Hawking's principal fields of research are theoretical cosmology and quantum gravity.
He wrote a series of books designed to explain the most difficult current theories to ordinary readers.
The first of them, A Brief History of Time, became a surprise bestseller, and was followed by three further popular titles.
His distinctive appearance and artificial speech have made him instantly recognisable world-wide, and he has never shirked the media spotlight.
He has appeared in such series as Red Dwarf, Star Trek: The Next Generation, The Simpsons and Futurama.
