Sir David Attenborough is to undergo heart surgery, promoters have said.
The veteran naturalist is having a pacemaker fitted and has cancelled a planned tour of Australia, the AFP news agency reported.
In a statement, Australian promoter Lateral Event Management said: "Sir David Attenborough is to undergo surgery in London today following advice from his cardiologist that he is in urgent need of a pacemaker.
"Sir David expressed his extreme disappointment as he said he was very much looking forward to coming back to Australia and he hopes to reschedule his tour in the near future."
The 87-year-old's Australian tour had already sold out.
His six decade-career includes a string of BBC smash-hit documentaries including 'Life on Earth', 'Frozen Planet' and 'Africa'.