"I think they are filming some stunts for the last scene. There is a big battle scene involving a lot of pyrotechnics and explosions.Update: News Of The World reports on the fire, stating that the set used in the last scene of the film was left badly damaged.
"There is a mocked up castle made of timber, steel and plastic - somehow it caught alight.
"By the time we got there the on-site fire crew had it pretty well under control - they did a good job.
"We just helped damp it down and inspected the area.
"I didn't see any cast - but there were about 100 crew, who were all evacuated to a marshalling are. They didn't seem too concerned.
"It's hard to say how much damage was caused, because some of it may have been planned as part of the filming.
"Perhaps they got much better footage than they expected."
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.