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The youngsters join John Lithgow as Hogwarts headmaster Albus Dumbledore, the school's key professors being played by Janet McTeer as Minerva McGonagall and Paapa Essiedu, as Severus Snape.

Frost has been cast as Hagrid, McTeer will play Minerva McGonagall, and Essiedu will portray Severus Snape.

In her later years, she was the imperious Dowager Countess of Grantham in Downton Abbey, and Minerva McGonagall in the Harry Potter films.

Dame Maggie Smith was famous not only for her impeccable comic timing but also for the caustic put-downs used with such stinging effect by characters including Downton Abbey's Lady Grantham and Harry Potter's Professor McGonagall.

In the Harry Potter films, she played the acerbic Professor Minerva McGonagall, famous for her pointed witch's hat and stern manner with the young wizards at Hogwarts.