The little princess is in the middle of her first full week at school, and a royal expert has revealed what they called her at nursery.

A royal expert told Entertainment Tonight that she was nicknamed "Warrior Princess" in light of her "feisty" behaviour.

On Monday, Prince William and Kate Middleton's 4-year-old daughter started school.

She waved and smiled at photographers as her and her 6-year-old-brother Prince George, who is beginning his third term, strolled through the gates of St Thomas' day school in Battersea in south London.

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At school, she will be known as Charlotte Cambridge, the same surname her big brother goes by.

Her nursery nickname, however, was much more fun, as Katie Nicholl explained to Entertainment Tonight.

She developed the nickname at Willcocks Nursery, in Kensington Palace, which she attended for 18 months.

Princess Charlotte arrives at school on Monday with her brother Prince George. Photo / AP
Princess Charlotte arrives at school on Monday with her brother Prince George. Photo / AP

Nicholl shared with Entertainment Tonight: "The other nickname that she had when she was at Willcocks Nursery, which was up until just before the summer, was Warrior Princess.

"I'm wondering if that might be a nickname that will stick with her. Apparently she earned that nickname because obviously she is a Princess, but she's quite a tomboy.

"She loves climbing trees and she's very much an adventurer, she was quite known for her feisty personality."

She suggested the moniker could stick with her at school, however relatives and friends call her "Lottie".

The princess often demonstrates her adventurous side and is said to love playing outside with her brother.

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She's also displayed her brave nature publicly before: at Prince Louis' christening she told reporters: "You're not coming".

The young royal loves to play outdoors. Photo / Getty Images
The young royal loves to play outdoors. Photo / Getty Images

Again, her cheeky side showed when she poked her tongue at the public at The King's Cup last month.

She did the same thing at Prince Harry and Meghan Markle's wedding last year.

She's been caught on camera poking her tongue out more than once. Photo / AP
She's been caught on camera poking her tongue out more than once. Photo / AP

As she stepped out for her first day at school, royal fans gushed over the little princess who looked adorable in her new blue and red uniform.

A video posted to the Kensington Palace Instagram account also captured the little princess flicking at her hair as she walked with her parents. A user commented: "Charlotte is going to be a rebel. She's trying to take out her pony[tail]."

READ MORE: • Prince William reveals Princess Charlotte 'loves unicorns'

She was also spotted with a cute unicorn accessory attached to her school book bag. Prince William later shared that the mythical creature is her favourite "animal".

"My daughter loves unicorns. Loves them. Very cool."