Holm Sound was written and composed by Mr Cooper for his mother, Charlotte. It is understood to be a piece that they enjoy playing together at home.

The Steinway piano is adorned with festive garlands featuring red berries, with mother and daughter sharing loving glances and smiles as they play for the full three-minute video.

Wearing a navy blue long sleeved top with a scalloped collar, a tartan skirt and a sparkly gold bow, Charlotte plays the instrument with confidence, switching her attention from the music to where her hands are on the piano.

As the calming music plays in the background, Catherine, wearing a long dark green velvet dress with puffed sleeves, delivers a voiceover as footage of guests arriving for the service on December 5 is played.

The princess reads the letter that was given to guests at the Abbey who attended the service. The personal message, given to the 1,600-strong congregation, sees Catherine speak about the importance of acts of kindness in times that can seem “fragmented or uncertain”.

“At its heart, Christmas speaks of love taking form in the simplest, most human ways,” she says. “Not in sentimental or grand gestures, but gentle ones. A moment of listening, a word of comfort, a friendly conversation, a helping hand, presence.”

Videos show guests hugging as they arrive outside Westminster Abbey and write their names on a loop of red paper to hang on the ‘connection tree’ outside the church’s great west door.

The sweet film featured as part of the opening segment of the programme, which broadcast on ITV this evening.

The Princess is understood to have wanted to film the duet to showcase connection and the values she spoke about in the letter given to guests at the Abbey and Community Carol Services. It was filmed at Windsor last week, with the composer able to join them and give them some guidance in between takes.

On witnessing Catherine and Charlotte sharing the stage together for their joint performance, Mr Cooper said: “It was just lovely. I think I was there for a little bit of encouragement. I mean, imagine anybody performing a piece of music in front of a film crew, and then the person who wrote it. Princess Charlotte played beautifully, she really has a wonderful way of playing the lighter notes just beautifully. So we were able to celebrate in that together.

“It’s certainly one of my favorite winter solstice moments. To walk into the Castle, to be greeted by a Steinway grand piano, and then to have a really poignant recital and performance. It was a very special, sort of quiet moment. There’s a power in reaching out to people, and that’s what they did here.”

Catherine played the piano at her 2021 carol service, as she accompanied musician Tom Walker. She has previously mentioned that her daughter has been learning and practicing the piano.

The Princess of Wales first wrote to Mr Cooper to express her love of his music and thank him for his dedication to promoting the beauty of the natural world through his craft.

Through their exchange of messages, the future queen revealed how she had enjoyed teaching Charlotte his compositions at home, creating a special bond between them.

As she planned to create a special moment as part of her fifth annual Together at Christmas carol concert, celebrating hundreds of community heroes at Westminster Abbey, the princess requested to play one of her favourite pieces with her daughter as part of the television on ITV1 on Christmas Eve.

“I’m always in the head and hearts and back gardens of others. I often think, in this case, this little piece of music sort of landed on her shoulder like a bird,” Erland said.

“And when you write a piece of music and you share it with the world, you’re sort of letting that out, and you’re letting it go, and you have no idea how far and wide it might fly. So it’s always a great surprise when people write to me or call me, and this was, you know, one of those great surprises. I didn’t expect it at all.”