The King and Sir Elton John have recorded an hour-long special episode of the singer’s Apple Radio show Rocket Hour, which will air on the monarch’s 78th birthday on Saturday.

Charles and Sir Elton sat down in secret at the Apple Music Studios in Battersea Power Station in south London last month for the musical collaboration.

The conversation for Sir Elton’s regular show was held to highlight the key role of music in the work of the monarch’s charity The King’s Trust, which is celebrating its 50th year.

The King described it as “an opportunity too good to resist”, while Sir Elton admitted afterwards: “I learnt a lot from speaking to His Majesty.”

Charles also thanked Sir Elton for adding “a little Rocket Power to the proceedings” and praised the pop star for becoming “such a national treasure after all these years”.

Sir Elton quipped “I’ll be on the Antiques Roadshow”, with the King laughing and retorting “Join the club”.

The pair shared memories of performances by some of music’s biggest stars in support of the trust over the last five decades – from Wembley Arena rock galas of the 1980s through to pop concerts in Hyde Park and sold-out charity shows at the Royal Albert Hall.

The episode also highlighted some of those who received help from the trust in their early days, including Muse, who were given a grant from the-then Prince’s Trust to purchase PA equipment when they were starting out as a band under the name Rocket Baby Dolls.

Sir Elton told the King: “I never knew that some of these musicians were involved in the trust. It’s great news to me… and it reinforces for me all the great work that you’ve done all these 50 years, because I had no idea. So bravo.”

The King said it was “wonderful” to see the difference the trust’s programmes can make, adding: “Well really, what I was trying to do was invest in the future, but particularly investing in talent.”

Sir Elton said: “That’s the greatest thing you can do. Young people are our future, and you’re doing a wonderful job.”

The disc-jockey duo were pictured together in the studio, standing side by side, with Sir Elton wearing his trademark colourful thick-rimmed glasses – this time in green.

Charles and Sir Elton took part in the episode – the first to be recorded in the new Apple Radio studios – in October when they both visited Battersea Power Station for the launch of the Apple x King’s Trust Creative Labs which aim to help young people explore potential careers in creative industries.

In a trailer, the King and Sir Elton are shown chuckling together as they chat during the recording.

Sir Elton said in a voiceover: “Hi I’m Elton John. Welcome to my Rocket Hour. This week, my special guest, and I really mean special guest. I’m so excited to hear what he’s got to say about music, about The King’s Trust and memories that we both share.”

The full list of their chosen tracks is being kept secret until the programme airs at 9am on Saturday, but it was revealed that Raye’s Joy, Skye Newman’s Hairdresser and Muse’s rendition of Feeling Good were among them.

In the episode, the King said: “Music has always been at the heart of The Prince’s, and now The King’s Trust.

“So when I was asked whether I would like to showcase a selection of music from among the many talented performers who have so generously supported us over the years, together with some of the newer artists the trust has helped to launch, it was an opportunity too good to resist.”

Charles is celebrating his birthday just over a week after he returned from a busy tour to the Caribbean.

The venture is his second project with Apple Music, after he took part in the King’s Music Room online radio show last year when he was asked to showcase 17 of his favourite songs.

Announcing news of the episode, a Buckingham Palace spokesperson said: “You could say this new opportunity therefore presented a slight challenge – how to make a similar impact and avoid what some might call ‘the difficult second album’.”

They added: “The answer was to find a world class musical collaborator.

“This year, therefore, I’m pleased to announce that the King is returning to Apple Music alongside, drum roll please… The Rocket Man himself, Sir Elton John.”

They described it as “a show that is something of shared musical journey” and said the King and Sir Elton’s playlist “spans a range of decades and genres, from some meaningful personal favourites and golden oldies, to this year’s new releases”.

“From Tina Turner and Rod Stewart to Calvin Harris and this year’s Mercury Prize winner Raye, we hope there is something there for everyone, whatever their age or musical taste,” the spokesperson said.

Sir Elton has long been a supporter of The King’s Trust, and performed at some of its rock galas at Wembley in the 1980s and donated a pair of his rose-tinted spectacles to raise money as part of its 50th anniversary auction.

The King’s Trust, then known as The Prince’s Trust, was set up by Charles in 1976 to help disadvantaged young people overcome challenging circumstances, get into employment or launch their own businesses.

The full episode of Elton John’s Rocket Hour will premiere at 9am on Saturday on Apple Music 1.

There will be additional opportunities to hear it for free on Apple Music 1 on Saturday at 5pm and 10pm, and on Sunday at 2am and 6am, and on both Apple Music 1 and Apple Music Hits at 10am and 2pm on Sunday.

It will then be available to listen to or download from the Apple Music platform.