Lady Gabriella was described as being a “real member of the carol service team’ and was influential in choosing the musical performances.
With experience as a singer-songwriter, Lady Gabriella is understood to have brough expertise to the concert which saw singers such as Paloma Faith and Olivia Dean perform.
An insider said she “enjoyed her role within the service and was excited to be a part of it,” whilst another described Kate as “incredibly grateful for her contribution.”
A friend said: “She was very touched and grateful to the Princess to be asked to contribute to her very special concert. She felt honoured.”
Lady Gabriella arranged for music by Restore the Music, a youth charity at which she is an ambassador, to be played as guest queued for entrance.
Polly Moore, CEO of Restore the Music, said: “Lady Gabriella is incredibly passionate about music and helping young people find their voice.
“She has been an ambassador for us for the past five years and often attends our events.
“When she messaged to ask if I would be interested, I said, “Are you kidding, of course!”
“She was incredibly approachable and a brilliant sounding board in the planning.”
The service, filmed on December 6, was the fourth ‘Together at Christmas’ organised by Kate and the first since publicly announcing her cancer diagnosis.
Her attendance at the event was also one of the few times that Kate has been seen in public since her illness.
The broadcast will open with the princess delivering a message in which she will urge the public to ‘slow down and reflect on the deeper things that connect us all.’
She will say: “Christmas is one of my favourite times of the year, it is a time for presents, tinsel and mince pies, but it’s also a time to slow down and reflect on the deeper things that connect us all.
“It is when we stop and take ourselves away from the pressures of daily life, that we find the space to live our lives with an open heart, with love, kindness and forgiveness, so much of what the Christmas spirit is all about.
“The Christmas story encourages us to consider the experiences and feelings of others. It also reflects our own vulnerabilities and reminds us of the importance of giving and receiving empathy, as well as just how much we need each other in spite of our differences.
“Above all else, it encourages us to turn to love, not fear.”