King Charles appears on Elton John's radio show in secretly-recorded birthday episode

King Charles III has appeared on Sir Elton John's radio show in an episode set to coincide with the monarch's 78th birthday.

During the Rocket Hour show, the King said the pop star had become "a national treasure after all these years", to which the singer joked: "I'll be on the Antiques Roadshow."

"Join the club," the King replied.

The pair discussed some of their favourite musical performances, as well as the role music has played in the King's charity, the King's Trust. The show was recorded in secret last month and will air this Saturday.

Sir Elton has long been a supporter of the trust, most recently donating a pair of his rose-tinted glasses to its 50th anniversary auction this year.

In a trailer for the upcoming episode, the King and Sir Elton are seen laughing together as they chat.

In a voiceover, the singer said he was "excited to hear what he's got to say about music, about the King's Trust and memories that we both share".

The King's Trust - previously known as The Prince's Trust - was established by Charles in 1976 to help young people from disadvantaged backgrounds overcome challenging situations, become employed or launch their own businesses.

During the radio episode, the King called it "an opportunity too good to resist" when he was asked to showcase music from the "talented performers who have so generously supported us over the years, together with some of the newer artists the trust has helped to launch".

The full list of their chosen tracks is under wraps until the episode airs but Skye Newman's Hairdresser, Raye's Joy and Muse's rendition of Feeling Good are said to be among them.

The pair will reminisce about some of the most memorable performances for the trust, from the Wembley Arena rock galas of the 1980s - which Sir Elton participated in - to charity shows at the Royal Albert Hall and pop concerts in Hyde Park.

The episode was recorded at Apple Music Studios - which hosts the show - in Battersea Power Station when the King and Sir Elton visited for the launch of the Apple x King's Trust Creative Labs.

The mentorship programme aims to help young people explore careers in creative industries.