John Lithgow's casting as Dumbledore confirms Harry Potter series will break major JK Rowling rule

John Lithgow's casting as Dumbledore in the upcoming Harry Potter TV series has already broken a major rule of JK Rowling's when it comes to bringing the Wizarding World to life.

After months of speculation, Lithgow seemingly confirmed that he would be taking on the role of the Hogwarts headmaster in an interview with ScreenRant, saying he was 'very excited' for the challenge.

"It was not an easy decision because it's going to define me for the last chapter of my life, I'm afraid. But I'm very excited," Lithgow said, adding that he'll be 87 at the end of the series.

Lithgow announced he would be playing Dumbledore in a recent interview (Jamie McCarthy/Getty Images)

The news is yet to be confirmed by series creator HBO, who told the outlet 'will only confirm details as we finalise deals', however, Lithgow's confirmation follows months of speculation that he'd been linked to the role.

Fans have also been quick to point out that Lithgow playing the Hogwarts professor will break one of the rules which author JK Rowling had the adaption of the films — which was to have a cast of exclusively British actors.

The rule was confirmed by casting director Janet Hirshenson, who previously told Huffington Post in an interview that Robin Williams had been turned down due to his American nationality.

"The 'only British' rule was so important that Robin Williams was even turned down to play Hagrid…Robin had called [Columbus] because he really wanted to be in the movie," he explained.

"But it was a British-only edict, and once he said no to Robin, he wasn’t going to say yes to anybody else, that’s for sure. It couldn’t be."

The role of Rubeus Hagrid would famously go to Robbie Coltrane.

However, eagle-eyed viewers will be quick to point out the '100% British rule' wasn't exactly accurate, given the fact that numerous stars, including Evanna Lynch, Domhnall Gleeson and Devon Murray, all hail from Ireland.

Albus Dumbledore was previously played by British actors Michael Gambon and Richard Harris (Warner Bros)

Two US stars would also end up making it into Harry Potter, with Eleanor Columbus and Verne Troyer playing the role's of Susan Bones and Gringotts Bank goblin Griphook respectively.

Zoë Wanamaker, who plays the role of Quidditch teacher Madam Hooch, was also born in the States but grew up in the UK.

Griphook would end up being voiced by Filius Flitwick actor Warwick Davis to get around the US accent issue, while Columbus - who is the daughter of director Christopher Columbus - was limited to a non-speaking role. She would later be played by Emma Jayne-Corboz in Harry Potter and the Deathly Hallows.

Columbus would later open up on his daughter's brief cameo in the film, revealing that she worked for around 80 days.

"She worked about 80 days. But she never spoke because you know the rule was if you’re not British, you can’t speak," he said.

Fortunately, Lithgow will have plenty of experience to draw upon when getting into character, as he previously used a British accent, while playing Winston Churchill in The Crown.

LADbible has reached out to HBO for comment.