Crews were first called to the scene shortly before midnight
Firefighters are tackling a blaze at Warner Bros studios.
Crews were called to the site in Leavesden, Hertfordshire, at 23:29 BST on Wednesday.
The council said the set involved was not being used at the time and there had been no reported injuries.
All eight Harry Potter films as well as other movies including James Bond, Fast and Furious and the Mission Impossible franchises have filmed at the studios.
A spokesman for Warner Bros would not comment on the blaze other than to confirm it was ongoing.
He said the public studio tours were unaffected and taking place as usual.
One of the fire engines can be seen parked by the perimeter of the studio site
Mark Hancock, who lives nearby, said: “My house is only about 200m away from the perimeter fence and we have a clear line of sight over gardens so we managed to see it out one of our windows.
“The smoke went high into the air, and as I watched, the smoke appeared to get thicker, so much so that we could see the flashing lights of the fire engines reflecting off the thick smoke.
“It was still going strong at about 1am and when I woke up this morning it was still smouldering.”
He added there was a “strong smell and taste of smoke in the air”.
At the scene: Alex Pope, BBC News
Smoke can be seen coming from the back of a studio building
As the studio is a closed set no-one can get close to the scene.
The studio backs on to a housing estate from where you can smell a faint burning odour.
A woman who lives there told me she was woken up by a strong smell at about midnight and her dog was barking and “he never barks”.
“The smell was so strong I thought it was coming from the house,” she said.
Another resident said she spoke to a firefighter this morning who told her it was “a significant fire”.
I can see smoke billowing from the back of the studios, but it seems like the studio is going about its day-to-day business.
Image copyright
Alamy
All eight films in the Harry Potter franchise were shot at the facility near Watford
A Hertfordshire Fire and Rescue Service spokesman confirmed 15 crews were sent to the scene when the fire broke out. There are now 18 crews tackling the blaze.
He said an aerial ladder was being used as well as 12 sets of breathing apparatus.
Police have also closed off Bridge Road in Leavesden to allow the fire service to run a hose from the nearby canal to the site of the fire.
James Bond and the Mission Impossible franchises have also been shot at the vast studios