PRINCESS Kate shared the special secret she told Prince Louis to keep from his sister, as the royal children joined their mother at the poignant Christmas carol service.
The Princess of Wales revealed the important task she gave her youngest son, in a conversation with singer Paloma Faith.
The annual star-studded Together At Christmas celebration kicked off tonight as the most important service yet, following Kate's cancer diagnosis.
Spirits were high as the princess said she had enlisted Prince Louis, 6, to keep something a surprise from his sister Charlotte, 9.
Kate shared how Princess Charlotte was unbeknownst about one of the most special acts of the night.
Seemingly talking about the soloists from the Royal Ballet who performed during the service, the princess opened up about how much the young royal loves ballet.
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Speaking to Paloma Faith, Kate said she asked Louis “Can you keep a secret otherwise it won’t be a surprise for Charlotte, who loves her ballet dancing."
Kate revealed that the young prince lived up to his tricky task.
The princess added: “He has kept the secret for, what, nearly two weeks now.”
Paloma Faith said: "Very challenging."
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Princess Kate replied: “For a six-year-old."
Despite the prince's good job, Kate revealed how he nearly broke at the last minute.
Kate added: “And then this morning he said 'Mummy, please can I tell her - I’m bursting'.
"I did drop off and he really wanted to tell her. I think - hopefully - he’s kept it.”
The young princess later looked delighted at the Royal Ballet's performance, suggesting Louis kept his promise to his mum.
The Princess of Wales also met singers Gregory Porter and Olivia Dean on the festive night.
She spent the service by her children as they held candles and sang along to Christmas tunes.
The royals were joined by more members of their family, including Princess Beatrice and Kate's sister Pippa Middleton.
Stars from across the world's of sports and TV also attended the special event.
Six-time Olympic gold medallist Sir Chris Hoy, who has terminal cancer, was in attendance.
Heartwarming notes from Prince George, Princess Charlotte and Prince Louis
Prince William beamed as he arrived with Prince George, Princess Charlotte and Prince Louis for the 'Together At Christmas' service.
The children looked thrilled to appear at their mother Princess Kate's Christmas carol service at Westminster Abbey this evening.
They were spotted carrying heartwarming hand-written notes that will be hung on the Kindness Tree.
The trio will be joined by other children outside to take part in the festive activity designed to express gratitude and love.
Princess Charlotte's sweet note read: "Thank you Granny and Grandpa because they have played with me".
The Sun revealed last month the children who survived the Southport tragedy in July will be attending too.
Kate also penned in the letter to guests how Christmas “encourages us to turn to love, not fear.
The love that we show ourselves and the love we show others.”
Olympian Adam Peaty OBE gave a speech at the festive event.
TV stars like Kate Garraway, Richard Madeley, Judy Finnigan, and Hannah Waddingham were also invited.
The Sun revealed last month that a very special invitation was extended to the children who survived the Southport tragedy in July.
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In letters to the 1,600 guests, Kate wrote how Christmas “encourages us to turn to love, not fear. The love that we show ourselves and the love we show others.”
Other guests include people in communities who have supported others through their work or volunteering.