Prince William has revealed his daughter, Princess Charlotte, is obsessed with one particular thing: unicorns.

Speaking during a visit to Harcome House, a charity for firefighters, Prince William said Princess Charlotte, who recently attended her first day of school, "loves unicorns."

THIS IS HOW PRINCESS CHARLOTTE WILL BE ADDRESSED AT SCHOOL

Princess Charlotte, 4, loves unicorns, according to her father, Prince William.

Princess Charlotte, 4, loves unicorns, according to her father, Prince William. (Getty)

"Loves them. Very cool," he said, according to People.

In a video posted by Kensington Palace last week of Princess Charlotte and Prince George, 6, on their way to school, their mother, Duchess of Cambridge Kate Middleton, was spotted carrying the beloved 4-year-old's backpack — which was equipped with a pink sequin unicorn keychain.

Princess Charlotte, formally known as Her Royal Highness Princess Charlotte of Cambridge and fourth in line to the throne, smiled and flicked her ponytail as she met with the lower school headteacher at Thomas' in Battersea in South London. Prince William said Princess Charlotte was "very excited" for her first day.

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In July, Princess Charlotte was also spotted posing for a photo with a fun unicorn accessory — a purse — at her brother Prince George's birthday party in July.