UPDATE: WB has responded to the reports with these statements, as provided by MTV:
In a statement, a “Potter” rep said of the incident: “It was a big special effects scene. Unfortunately, there was a small fire as a result of that. As a precaution, as we had already mobilized our own in-house fire team who had it under control, the fire brigade was called and it was put out in under an hour.”
The statement went on to confirm that Leavesden wasn't damaged at all by the fire and that the fire-plagued set was already going to be rebuilt anyway. “The scene was where the set was being destroyed and then it was to be rebuilt for a new scene, as part of a battle for Deathly Hallows part two.”
The British tabloid News Of The World is reporting that a HUGE blaze has wrecked the Hogwarts set after a battle scene went spectacularly wrong.
Explosives used in action sequences set light to scenery for the wizardry school.
Firefighters battled for 40 minutes to bring the flames under control but the set – centrepiece for the film's Battle of Hogwarts climax – was left badly damaged.
Our source on set said: “We were shooting the scenes where Hogwarts gets blown up during the battle. But fire managed to catch hold of the set and it went up in flames. It was completely out of control at one point.”
Some actors were still filming at the studio but none of the movie's biggest stars – Daniel Radcliffe (Harry), Emma Watson (Hermione) or Rupert Grint (Ron Weasley) – was involved. No one was injured but our source added: “The special effects guys involved are devastated.
“There's always a risk when there's shooting involving fire and explosives but this shouldn't happen.”
The costly blaze will not help studio bosses, who are battling to finish the film in time for its November release.
We'll let you know as soon as we know anything more. Thanks to loony_luvgood for the tip.