Kate Middleton, 39, and Tom Walker, 30, collaborated for ITV's Royal Carols: Together at Christmas which aired on Christmas Eve. Brits were left stunned as they watched the Duchess of Cambridge play the piano alongside the Scottish-born musician at the festive event.

       Following her performance, Mr Walker's single lept up the charts.

       He told the Sun: "It was so well received, I probably owe her royalties now.

       "She sent me a letter thanking me for the opportunity but it should be me thanking her.

       "The song has jumped 57 places in the charts. It was a wonderful experience and she was so warm.

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       "She made the effort to thank all the musicians but it was us who felt so grateful to be part of something so special."

       Mr Walker also explained how the pair prepared for the televised event.

       "We met for a rehearsal beforehand and I gave her a recording of the track without the piano part so she could practise.

       "It was obvious she had taken the time to perfect it.

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       "She was brilliant.

       "You think the Royal Family almost don’t get nervous, they’re so used to the spotlight.

       "But this was obviously a very different, new experience for her."

       The duo's previously crossed paths at a charity event in support of Action on Addiction.

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       At the event, Mr Walker performed his hit single Leave A Light On.

       While Kate was reportedly nervous ahead of the performance, users on social media praised the Duchess for her piano playing skills.

       One wrote on Twitter: "Beautiful. My mother is in tears of joy and all 3 generations of our family are saying the royal family is safe in her hands. Catherine’s parents must be bursting with pride!"

       Another said: "A big thank you to Tom Walker and the Duchess of Cambridge. This was magnificent. Absolutely loved every second."

       A third added: "What a beautiful moment. As usual, the Duchess of Cambridge was poised, elegant and played beautifully. I loved her smile at the end!"