Catherine, Princess of Wales and Princess Charlotte "didn't need direction" for their piano duet because they were "so good"

Erland told BBC Breakfast: "I'd had a chat with the princess earlier in the year and I knew she enjoyed playing this piece of music with family.

"It was a big surprise to be invited to Windsor Castle and hear it performed by a pair of princesses. I didn't need to give direction, they were so good.

"Performing in front of a film crew and the person who wrote the music is a nerve-wracking thing for anybody, I'm sure. They played it several times and then they asked me to have a wee go as well, and I gave a few pointers. But I didn't need to. I think I was there as a cheerleader."

The Scottish musician praised Charlotte as "very confident".

He added: "She played it so beautifully."

Erland wrote Holm Sound for his mother, who is also called Charlotte, and the "full circle moment" of family ties.

He said: "This is the wonderful coincidence, it's about how we see our parents in our children's eyes and but then when you look at your mother and you look at granny, you see your children. It's about that sort of full circle moment, and it's about motherhood."

Catherine and her daughter recorded the duet in Windsor Castle's Inner Hall earlier in December, with the older princess playing with her left hand and Charlotte with her right.

As the performance aired, footage was shown of guests arriving at the Westminster Abbey carol service, including Catherine and her husband, Prince William, watching as their three children tied paper chains with their names on to a "connection tree".

The Wales family were joined at the carol service by other members of the royal family including Zara and Mike Tindall, Sophie, Duchess of Edinburgh, and Prince Michael of Kent, as well as Catherine's parents, Carole and Michael Middleton, and her brother James Middleton, who was accompanied by wife Alizee.