Celebrated physicist Professor Stephen Hawking has been forced to cancel a number of public engagements because of ill health, his university has confirmed.
They include this year's prestigious Reith Lectures, which have been postponed by the BBC.
A spokeswoman for Cambridge University, where the 73-year-old cosmologist is director of research in the Department of Applied Mathematics and Theoretical Physics, said: "Prof Hawking is not very well and a few public appearances have had to be cancelled."
She could not say anything more about the condition of the professor, who is thought to be the longest living survivor of motor neurone disease.
Prof Hawking had been due to record the Reith lectures at the Royal Institution in London on Thursday evening. They were due to be broadcast on BBC Radio 4, starting on Tuesday November 24.
In his talks, the professor planned to describe the nature of black holes and answer questions from BBC Radio 4 listeners.
A BBC spokesman said: "Unfortunately, Thursday's recording of the BBC Reith Lectures with Professor Stephen Hawking is no longer going ahead as he is unwell.
"We are postponing the broadcast of the lectures on Radio 4 and are liaising with Professor Hawking and his team about the next steps once he is better."