Princess Kate thanked Katie Melua moments after the singer performed during the Together at Christmas Carol Service.
Ms Melua, 41, sang White Christmas as she stepped up at Westminster Abbey.
The singer said: "It was my first time performing 'White Christmas', thank you to Simon Haw for the brilliant new arrangement.
"And thank you to the Princess of Wales for having me, it was unbelievably special to be a part of it."
The singer performed White Christmas at the service | GETTYTRENDING
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Following her conclusion of the song, Kate appeared to mouth the words "thank you" towards Ms Melua as the service moved on.
Her performance in the broadcast on ITV1 and ITVx came moments after Princess Kate and Princess Charlotte played a duet piece on the piano.
The segment at the beginning of the programme featured the mother-and-daughter duo playing Holm Sound, by Scottish composer, producer and multi-instrumentalist, Erland Cooper.
The performance was accompanied by a voiceover from the Princess of Wales reading the letter handed to guests at Westminster Abbey.
Ms Melua appeared to be thanked by Princess Kate following the performance
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Mr Cooper joined the duo on the day of the performance and provided some guidance.
The composer's work is inspired by nature and connection, themes which resonate deeply with Kate.
Holm Sound was written and composed by Mr Cooper for his mother, Charlotte.
The Princesses often play the beautiful piece together at home, with the special moment being recorded in the Inner Hall at Windsor Castle.
Princess Kate and Princess Charlotte played a special duet | ITV/BBC STUDIOSBBC Studios Events and The Royal Foundation co-produced the programme, which included two special films showcasing individuals and organisations who have demonstrated love and compassion to others during difficult times.
Kate has played the piano at previous festive concerts in the past, including in 2021, when she put on a heartfelt performance at a Westminster Abbey service which aired on TV.
During that appearance, she amazed onlookers by playing the piano to support singer Tom Walker with a moving song.
The pair played Mr Walker's Christmas single For Those Who Can't Be Here together, with Kate admitting that she felt "quite nervous".
The pair played Holm Sound by Erland Cooper | ITV/BBC STUDIOSAt the Carol Service, which took place on December 5, the Princess of Wales was joined by the rest of her family for the service, with Kate sitting next to her youngest child, Prince Louis.
Prince William and Prince George sat furthest from the left as the family joined in with the carols.
Also in attendance were Zara and Mike Tindall, as well as Sophie, the Duchess of Edinburgh, who sat on Kate's left.
A repeat of the show will be broadcast on Christmas Day morning.
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