Elle is the 22nd great-granddaughter of King Edward III, who ruled England for nearly 50 years from 1330 to 1376. Kate's mother Carole Middleton is a 21st great-granddaughter, linking Elle, and her sister Dakota, and Kate as distant relatives.
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"Generation after generation, the lines we looked at pieced back directly to King Edward III proving that Elle is a direct descendant of royalty," said Michelle Ercanbrack, a family historian for Ancestry.com.
Elle Fanning and the Duchess of Cambridge
The online family history website searched through over 700 years' worth of documents including birth and death certificates, and census records.
Speaking to People magazine, Michelle added: "You can consider her a long lost princess. This connection is so unique and rare."
It's a tale of life imitating art for Elle, who is playing Princess Aurora in the upcoming Disney film Maleficent, a Disney retelling of the animated classic Sleeping Beauty.
Elle plays Aurora, the princess who is cursed to prick her finger and sleep for 100 years. In this new version, out in the UK on Wednesday, Angelina Jolie plays Maleficent and we hear her side of the story.
Elle Fanning and Angelina Jolie
Of working with Oscar winning actress Angelina, Elle said: "It's kind of a dream to say that you even met her and let alone to be in scenes with her. It was really really cool."
"She really got into her character in a way. It was super exciting. Everything was so fun," Elle added to People magazine.
Angelina and Brad Pitt's youngest daughter, Vivienne, makes her debut acting role in the Disney film, but Angelina explained at the London press conference that the job was out of necessity.
"The reason we ended up needing to put Vivienne in the film is because all the children we brought to set, including crew members' kids, were terrified. They would cry or freeze." she said.