The surprise rule came to light last year, when Kate visited the Chelsea Flower Show. During a chat with a group of inquisitive children, she revealed one thing she is not allowed to provide the public with as a souvenir – despite it being a relatively simple request.

At one point, Kate and the kids looked around the Royal Entomological Society’s garden, where they tried to identify the bugs there and drew some sketches. Keen to secure something to remember her by, some of the children asked the princess to sign her name on their drawings.

But according to The Telegraph, she told them: “My name’s Catherine. I’m not allowed to write my signature, it’s just one of those rules.” However, not wanting to leave the children empty-handed she drew some sketches for them – including one of a flower, another of a tree and one of a pond surrounded by some plants.

However, earlier this month, William admitted to breaking the royal rule as he visited a world-famous beach in Cornwall.

William signs the plaster cast of a young boy on the beach.

While touring Fistral Beach, he made one little boy’s day. Phoenix Kanes, nine, cheekily stopped the prince and asked if he would sign the plaster cast on his broken arm – and he obliged, telling him: “Shhh, don’t tell anyone. I’m not meant to do that!”