PRINCESS Kate shared the special secret gave 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 to her youngest son, in a conversation with singer Paloma Faith.
The annual star-studded Togther 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.
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."
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.