Firefighters attack a fire at Warner Bros. Studios.
The teams were called on the Leavesden, Hertfordshire site at 11:29 pm ET on Wednesday.
The board said that the whole involved was not used at the time and that there had been no reported injuries.
The eight Harry Potter films and other films, including James Bond, Fast and Furious and Mission Impossible franchises, were shot in the studios.
A spokesman for Warner Bros. would not comment on the fire except to confirm that it was underway.
He added that public studio visits were not affected and proceeded as usual.
Mark Hancock, who lives nearby, said: "My house is only 200 meters from the fence and we have an unobstructed view of the gardens, so we managed to see it through one of our windows.
"The smoke was rising in the air and, as I watched, the smoke seemed to get thicker, so that we could see the flashing lights of the fire engines reflected in the thick smoke.
"It was still strong around 1 am and when I woke up this morning, he was still smoking."
He added that there was "a strong smell and a taste of smoke in the air".
On the scene: Alex Pope, BBC News
As the studio is a closed plateau, no one can approach the stage.
The studio is leaned to a subdivision from where we can smell a slight burning smell.
A woman who lives there said that she had been awakened by a strong odor around midnight and that her dog was barking and that he was "barking" ever again.
"The smell was so strong that I thought she was coming home," she said.
Another resident said that she had talked to a firefighter this morning who had told him that it was "a major fire".
I can see smoke escaping from the back of the studios, but it seems like the studio is dedicated to its daily activities.
A spokesman for the Hertfordshire Fire and Rescue Service confirmed that 15 crews had been dispatched to the scene when the fire broke out. There are now 18 teams tackling the fire.
He added that an aerial ladder was used as well as 12 respirators.
Police also closed Bridge Road Road in Leavesden to allow the fire department to pass a pipe from the canal near the fire site.