Deteriorating facial muscles may make Professor Stephen Hawking to lose his distinctive computerised voice, as it has slowed down his ability to speak to one word per minute.
The world's most famous physicist uses technology operated by twitching his cheek to painstakingly write words on a computer.
A tiny infrared sensor on his glasses detects his cheek pulses, and then selects words displayed on a linked-up computer screen. The chosen words are then spoken by a voice synthesiser.
When the 69-year-old theoretical physicist and cosmologist started using the system, he could write 15 words a minute but the motor neurone disease that has left him wheelchair-bound for more than 40 years is advancing, meaning he is often unable to compose more than a couple of paragraphs of text in an hour.
Although faster means of communicating may be available, the Cambridge University academic is concerned he could lose the electronic voice he has had for a quarter of a century.
"His speech has got slower and slower and on a bad day he can only manage about one word a minute," the Daily Mail quoted Judith Croasdell, his personal assistant as saying.
"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," Croasdell said.
Hawking, the best-selling author of 'A Brief History of Time', caught pneumonia in 1985 and needed a tracheotomy that left him unable to speak.