He recently declared he thinks he will live past 100, and already has a "pretty full" schedule for this year.

And 91-year-old Sir David Attenborough says he will only stop working when he "can't walk up and down steps any more" or hands in "substandard" work, according to the Daily Mail.

The veteran broadcaster and naturalist, who has four major projects on the go, said the thought of not working fills him with "dread" but he is currently "all right".

For his latest BBC documentary, about the 200-million-year-old fossil of a sea dragon, he said he was asked to walk up a spiral staircase several times in order to get the right shot.

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He told 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."

He added: "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."

Attenborough, who narrated the recent hit series Blue Planet II, has long fielded questions about when he will slow down or retire completely, but has maintained he will keep going for as long as he is offered work.

However, he hinted to the magazine that he would be quick to step down himself if he noticed he was not delivering the standard of work he expects of himself.

He said: "I would like to think I would be able to detect when I couldn't find the right words any more. 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… If I thought I was turning in substandard work, that would stop me."

Ahead of Blue Planet II, he revealed his ambitions to live beyond 100. When asked if he thought there was any reason he couldn't do so, he replied: "None whatsoever."

This year, in addition to Attenborough And The Sea Dragon, which starts this Sunday at 8pm on BBC One, he said he already has a 'a couple of Natural Worlds, an overseas project and another big series' in the pipeline.