The 91-year-old broadcaster and naturalist has also said the thought of not working fills him with "dread".

Sir David revealed that, in his new documentary, he was asked to walk up a spiral staircase several times in order to get the right shot.

He told the Radio Times: "I did it at least six times because of these bloody directors! 'Could you turn a little sooner? Could you look over here? Could you walk down the steps, as well as up?'

"If I can't walk up and down steps any more, that will stop me.

"Yes, I do dread not working, although there are things I can do without running up steps six times - books to be written, things I've never got round to."

The Blue Planet II creator added: "But at the moment it seems to be all right."

Sir David, whose career has spanned more than six decades, said while he does not like to think about retiring just yet, he "would like to think I would be able to detect when I couldn't find the right words any more".

He said: "If I think I'm not producing commentary with any freshness, or which is apposite or to the point, I hope I would be able to recognise it before someone else told me.

"I spend a lot of time fiddling with the words. I write a commentary, and feel it's finished, then go back over it the next day and find it full of infelicities, clumsiness and redundancies."

He added: "If I thought I was turning in substandard work, that would stop me."

Sir David's new documentary, Attenborough And The Sea Dragon, airs on Sunday at 8pm on BBC One.

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