The singer, 30, performed his previously unheard Christmas song, For Those Who Can’t Be Here, accompanied by Kate playing the piano, in the concert which was broadcast on ITV on Christmas Eve.
The Duchess of Cambridge hosted and spearheaded the Royal Carols: Together At Christmas service, which paid tribute to the work of “inspirational” people who served their communities during the Covid-19 pandemic.
Speaking to Rev Kate Bottley on BBC Radio 2’s Good Morning Christmas after the broadcast, Walker said he had kept the performance a secret, adding: “Even from my own mother. She was in floods of tears watching it on the telly, it was beautiful.”
He revealed he had been asked to perform at the festive community carol service after performing in front of The duchess at a previous charity event.
He told Ms Bottley: “She was doing a speech and she heard one of my songs, Leave A Light On, and she asked if I could come down and play at the carol service and I said yeah that’s no problem, but I’ve also written a Christmas song that no-one’s heard yet, I can send it over and you can have a listen.
“So I sent it over to the team and they absolutely loved it, and then a week later they came back and said ‘Um, would it be alright if the duchess played along with you’ and I was like ‘What?’”
He said of Kate, who gained her grade three piano and grade five theory: “She’s amazing. We had the rehearsal a couple of weeks before just to make sure we were both happy with all the parts and everything and she was amazing.
“She picked it up so well, it’s really not easy to go from not playing with other musicians for like 10 years to jumping straight in with like a whole band you’ve never met and camera crews doing live takes that’s being filmed in Westminster Abbey so I thought it just went so well and yeah she’s (a) very talented musician.”
Asked where he goes from here, and who he will perform with next, he joked: “That’s it now, I’ve probably peaked.”
The singer said he was spending Christmas Day with his family and that early next year he will be finishing his album.
He said: “I’m literally heading straight into the studio as soon as I can get back, so I think from the 10th January onwards, Coronavirus permitting, we’ll be in the studio together finishing off the album, so yeah, super excited, and then we’ll have a tour at the end of the year, a couple of festivals. So yeah, I’m excited to get back to it.”
The Royal Carols: Together At Christmas service was attended by those Kate and William had spent time with during recent engagements, as well as members of the armed forces involved in Operation Pitting to evacuate British nationals and eligible Afghans from Afghanistan, young carers, faith leaders and those who may have been more vulnerable or isolated during the pandemic.
It also included a performance from Ellie Goulding and featured readings from Harry Potter star Tom Felton and TV presenter Kate Garraway.