Sir David Attenborough has said he will retire from broadcasting if he feels his work has become substandard or if he can longer walk up and down stairs.

The 91-year-old, who narrated the UK’s most watched television programme of 2017, Blue Planet II, said his schedule for 2018 was already looking “pretty full” but he would stop working if he felt his commentary had lost its way.

“I would like to think I would be able to detect when I couldn’t find the right words any more,” Attenborough told the Radio Times. “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.

“If I thought I was turning in substandard work, that would stop me.”

Attenborough, who started working full-time for the BBC in 1952, said he would also stop if physical problems impeded him.

In his latest documentary, Attenborough and the Sea Dragon, he explores a fossil discovery on Dorset’s Jurassic Coast and at one point he is seen rushing up a spiral staircase.

“If I can’t walk up and down steps any more, that will stop me,” he said. “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. But at the moment it seems to be all right.”

This is thought to be the first time Attenborough has spoken about the prospect of retirement. In the past he has brushed away questions about when his career may end.

As well as Blue Planet II, other BBC programmes that Attenborough has worked on include Planet Earth and Life on Earth. He is to lead another big BBC series this year, details of which have yet to be revealed.

Blue Planet II was named the best television programme of 2017 in a poll of TV critics by the Radio Times. The Guardian ranked it as the fourth best show of 2017.

The series aired from October to December, 16 years after the original Blue Planet was shown on the BBC. It has been sold to more than 3o countries.

Attenborough said he was lucky to be associated with programmes about the natural world.

“It is extraordinary to think that everyone who has reached the age of 75 will have seen programmes of mine throughout their lives,” he said. “People write and say nice things. What I do isn’t very controversial, because people love looking at the natural world and I’m the person lucky enough to be associated with that.”