Sir David Attenborough has written a letter to a four-year-old boy to reassure him that the human race need not become extinct if “we look after our planet properly”.

Otis Allen, from Cardiff, had written to the 94-year-old naturalist asking if the human race would become extinct one day “like the dinosaurs”.

Otis had asked the same question of his mum, Gerry Holt, as she was putting him to bed, and she said she was momentarily “stumped” by the sobering query.

They devised the plan to contact Sir David the next morning, sending him a card with a drawing of a dinosaur by Otis.

To the surprise of Ms Holt, Sir David’s “beautiful” reply arrived three days later, on Wednesday.

Sir David thanked Otis for his letter, responding to his extinction query: “The answer is that we need not do so as long as we look after our planet properly.”

“I was just staggered,” Ms Holt, 38, told the PA news agency.

“There’s something really special and poignant about a 94-year-old conservationist who has spent his life working to save the planet corresponding with a four-and-a-half-year-old who’s just at the very beginning of his journey to understand the impact of climate change and our behaviour on the planet.

“Otis was really thrilled – he’s really fascinated by venus fly traps at the moment so he’s been watching video clips of Sir David talking about them.

“His little face just lit up when I read the letter to him this morning… but I think he’ll need to be a bit older before he really understands the significance.”

Ms Holt, a communications officer at Cardiff University, described Sir David as a “wonderful human being” and a “national treasure”, adding that the letter “made us all smile”.

“He has that innate ability to inspire and engage people, whether it’s speaking at UN climate change talks or writing to a young boy about the dinosaurs,” she said.

“It’s thanks to people like Sir David that Otis will grow up knowing about the climate emergency we face – and I think we should all be so grateful for his work.”