King Charles III and Queen Camilla are among well-wishers to share a birthday message celebrating Sir David Attenborough turning 100.
The royal couple also shared photographs of Sir David, including one of him with a young Prince Charles and Princess Anne in 1958, in which he is introducing them to Cocky the cockatoo, from his BBC Zoo Quest TV series.
The King and Queen wished him a very happy birthday, adding: "Enjoy your special celebration this evening!"
The veteran broadcaster and environmentalist has said he was "completely overwhelmed" by messages he had received ahead of his big day, which includes a special concert on Friday evening at the Royal Albert Hall in London.
Sir David added: "I simply can't reply to each of you separately, but I'd like to thank you all most sincerely for your kind messages, and wish those of you who have planned your own local events: have a very happy day."
In a video for the Earthshot Prize, which celebrates climate leadership and innovation, the Prince of Wales said: "Happy 100th David, cannot believe it's your 100th birthday."
He went on to thank him for all his support, while noting how "everything you do continues to inspire me".
Prince William's brother, the Duke of Sussex, is also among the well-wishers, describing Sir David as a "secular saint" in an article in Time.com.
"His most significant contribution has been the systematic dismantling of the notion that climate issues are happening 'somewhere else'," he said.
"Young people continue to listen to him not just for the spectacle of nature, but for a sense of continuity in an unstable world."
Former England men's football captain Sir David Beckham simply called the broadcaster "our National Treasure", while actress and activist Joanna Lumley wished the broadcaster a happy birthday with a little help from the people of Stroud, Gloucestershire, in a video message.
TV naturalist and presenter Chris Packham wrote in The Big Issue: "I don't think that any person in the entire history of our species has made such a significant contribution to engaging people and developing a love for all of life on Earth as David Attenborough."
Meanwhile, the World Wide Fund for Nature (WWF) shared a birthday tribute video, voiced by actors Dame Judi Dench, Morgan Freeman, Miranda Richardson, Asa Butterfield, Sam Heughan and Iwan Rheon, along with former Spice Girl Geri Halliwell‑Horner and wildlife presenter Liz Bonnin.
It is a spoken-word version of the Louis Armstrong classic song, What a Wonderful World, featuring footage of various animals.
Oscar-winning composer Hans Zimmer also paid tribute, saying that despite his extensive feature film success, "none of it is as important as working for David Attenborough because that is really about the existence of our planet."
Actor Sir Ian McKellen added that Sir David "sums up what was best about the BBC" with "serious programmes made for a popular audience".
"His ability to communicate his own enthusiasms are very precious and he's brought such joy to so many people," he said. "And I think, along with a lot of people, my favourite television programmes are probably natural history."
Friday evening's show at the Royal Albert Hall is the climax of a week of special events and broadcast programming in honour of Sir David, who was born in 1926 and joined the BBC in 1952.
Presenter Kirsty Young will host the special 90-minute concert celebrating Sir David's life, which will air on BBC One and iPlayer from 20:30 BST.
Special guests including Sir Michael Palin, Steve Backshall, Liz Bonnin and Chris Packham will appear at the event to reflect on Sir David's life and legacy.
Ahead of the concert, Young said: "Sir David's gift to the world has been a life spent exquisitely revealing Earth's wonders to us all.
"The very least he deserves is a big 100th birthday bash at the Royal Albert Hall. I'm very happy indeed, as the host, to be able to invite everyone to the party."
The event will recall some of the most memorable wildlife moments from Sir David's career and the BBC's natural history archive.
Live music from the BBC Concert Orchestra will include pieces associated with his most famous television series, including the snakes and iguanas chase from Planet Earth II, and the wave-washing orcas sequence from Frozen Planet II.
The concert will also feature performances from Bastille frontman Dan Smith, who will join the orchestra for a rendition of the band's hit Pompeii, which featured in Planet Earth III.
Elsewhere, Sigur Rós will perform Hoppípolla, which was used in the promotion of Planet Earth and Planet Earth II, while other musical guests will include singer Sienna Spiro and harpist Francisco Yglesia.
The BBC has been celebrating Sir David's centenary with special programming throughout the week.
Sir David and members of his former production team reflected on the making of their groundbreaking 1979 series Life on Earth for a documentary broadcast last weekend.
Meanwhile, recent BBC One series Secret Garden saw Sir David examine the hidden worlds and wildlife thriving in British gardens. Many of his other programmes have also been made available as part of a dedicated collection on iPlayer.
The BBC's chief content officer Kate Philips said Sir David's 100th birthday marked an "extraordinary" moment, describing him as a "truly remarkable individual".
Sir David was born in west London on 8 May 1926, and has also fronted pioneering natural history series including his Life Collection, The Trials of Life and The Blue Planet.
He has two children with wife Jane, who died in 1997. His brother Richard was an Oscar-winning actor and director, and died in 2014.
On Thursday, the Natural History Museum paid tribute to Sir David by naming a species of parasitic wasp after him.
The Attenboroughnculus tau is native to the Patagonian lakes of Chile, and a specimen was recently found in the museum's collection, four decades after it was collected.
Other species to have been named after the broadcaster in the past include a wildflower, butterfly, grasshopper, dinosaur and ghost shrimp.
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