"Charlotte is the spitting image of the Queen!" one fan wrote.
While another social media user commented: "Princess Charlotte I have a feeling she's going to rule her brothers. Takes after her amazing gran!"
The side-by-side images of a young (then Princess) Elizabeth against a four-year-old Princess Charlotte show that the Windsor genes remain strong.
Just like with Prince George's first day of school, Kensington Palace shared official portraits from the big day.
Simon O'Malley, Headmaster at Thomas's Battersea, shared his excitement over Princess Charlotte's first day.
He said: "We are delighted that The Duke and Duchess of Cambridge have decided that Princess Charlotte will join her elder brother, Prince George, at Thomas's Battersea."
"We greatly look forward to welcoming her and all of our new pupils to the school in September."
That's not the only family connection at the school.
Charlotte will find a friend in the fellow pupil, cousin Maud Windsor - the five-year-old daughter of Lord Frederick Windsor and Sophie Winkleman.
She is in the same year as George.
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