Ralph Fiennes’s nine-year-old nephew has landed a part in the next Harry Potter film – playing a younger version of Fiennes’s character.

Hero Fiennes Tiffin will join his uncle and cast regulars Daniel Radcliffe, Rupert Grint and Emma Watson for Harry Potter and the Half-Blood Prince in which he plays Lord Voldemort, the villain, as an 11-year-old. Frank Dillane, the 16-year-old son of the actor Stephen Dillane, will play the teenage Voldemort.

The announcement, made yesterday, will disappoint thousands who turned up for casting. It also comes days after thousands of teenage girls who auditioned for the role of Lavender Brown, who has eyes for Grint’s character, Ron Weasley, discovered that Jessie Cave, a 20-year-old professional actress, had won the part.

Cave, who is due to appear in the new CBBC drama Summerhill and will appear soon on the big screen opposite Dame Helen Mirren in the film Inkheart, is two years older than the age stipulated at the audition in July. Producers had held an open casting call for girls aged 15 to 18 at Earls Court, London, and said: “No acting experience necessary.”

A spokeswoman for the production told The Times: “With the roles of Lavender and the teenage Voldemort, the casting director had someone in mind but felt they should have one more look.”

In Harry Potter and the Half-Blood Prince, J. K. Rowling has made Lord Voldemort a growing threat to both muggles and wizards, and Hogwarts is no longer the haven it once was. Potter suspects that the dangers may even lie within the castle, but Dumbledore, the Hogwarts headmaster, is more intent upon preparing him for the final battle that he knows is approaching.

Leading actors joining the cast, making their Potter debuts, include the Oscar-winner Jim Broadbent, whose films include Iris and The Chronicles of Narnia: The Lion, the Witch and the Wardrobe, who will play Horace Slughorn, the Potions Professor, who gives Harry an old potions textbook that had once belonged to the Half-Blood Prince.

The award-winning actress Helen McCrory, recently seen in Becoming Jane and The Queen, will play Narcissa Malfoy, mother to Draco Malfoy and sister to the evil Bellatrix Lestrange.

They will be reunited with returning cast members including Helena Bonham Carter, Robbie Coltrane, Alan Rickman and Maggie Smith.

Steve Kloves, who scripted the first four films in the franchise, is adapting the screenplay. David Yates, who directed this year’s summer blockbuster Harry Potter and the Order of the Phoenix, returns, and the producer is David Heyman, who has produced all the Potter films.

Filming has begun at Leavesden Studios, Watford.

— Harry Potter and the Half-Blood Prince will be released next year.

Magic numbers

£320m worldwide gross for Harry Potter and the Order of the Phoenix
£650m J.K. Rowling’s approximate earnings from books and films

Source: Times database

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