The Duke and Duchess of Cambridge cheated in a bid to outdo one another and impress TV Baking star Mary Berry.
Prince William enlisted the help of former Bake Off winner Nadiya Hussain to help him roll up a roulade while Mary was not looking, for his appearance A Berry Royal Christmas, which will air tonight.
Determined not to let her husband beat her, his wife Kate was then caught sprinkling icing on her own cake after their set time was up.
Before the bake-off, William claimed he had 'made hundreds of roulades, loads', adding: I'm going to write a book about roulades and it'll be a bestseller.'
He later admitted it was his first time making the dish.
Prince William enlisted the help of former Bake Off winner Nadiya Hussain to help him roll up a roulade - before his wife cheated on her own creation
The Duchess of Cambridge revealed she stays up until midnight baking her children's birthday cakes. Pictured: Kate and Mary decorating cakes at RHS Garden Wisley, Surrey
Judging the bake off diplomatically, Mary told the pair: 'It's a draw. I don't want to quarrel with either of you. Well done for being so sporting – I know you're both very competitive.'
Speaking in scenes that will be aired as part of Mary Berry's festive show, William also said he and the Duchess of Cambridge made an effort to teach Prince George, six, Princess Charlotte, four, and Prince Louis, one, about homelessness.
Kate Middleton also revealed Prince William tried to woo her with his cooking skills while they were at university together.
She also admitted one of her youngest son's first words was 'Mary' because the royal household is stocked with Mary Berry's cook books and said one of the last things she cooked with her children was pizzas using Mary's own dough recipe.