The King will mark his 78th birthday on Saturday with an hour-long special episode of Sir Elton John’s Apple Radio show Rocket Hour.

The programme, recorded by the pair in secret, will premiere at 9am that day on Apple Music 1.

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

The conversation 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.”

King Charles III and Sir Elton John sat down for an hour-long special of the singer’s ‘Rocket Hour’ radio show to mark the King’s 78th birthday (Getty)

Charles also thanked Sir Elton for adding “a little Rocket Power to the proceedings”.

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

Sir Elton said in a…