Kate Middleton will have to follow this silly royal rule as she celebrates her 37th birthday today

There’s no denying the Windsors follow a hefty rulebook ... from not showing public displays of affection and not eating garlic, to only wearing nude nail polish and keeping quiet on political opinions.

Yes, it's a lot to take in.

And now we've discovered another rule the royal's have to follow - and it’s all to do with their birthdays.

Today marks the Duchess of Cambridge's 37th birthday.

While we can currently only speculate as to how the mother-of-three will be celebrating her day of birth, there is one thing we know for sure she won't be allowed to do ...

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And that is eating a slice of her birthday cake before the Queen!

Yep, traditionally, the birthday boy or girl would get first dibs on a slice of cake.

But according to royal rules, no one is allowed to eat before the Queen – and this includes birthday cake.

Unfortunately for Kate Middleton, she is also not allowed to accept any gifts from people who aren’t friends or family.

Unless the Duchess has received a lovely bouquet of flowers, consumable items or copies of books, she will be forced to turn them away.