18 crews are tackling a blaze at one of the stages at Warner Bros studios after a fire broke out late last night (Wednesday).
Firefighters were called to the film studios in Leavesden at 11.29pm last night.
The council said the set was not being used at the time and there had been no 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.
Public studio tours are reportedly unaffected and taking place as usual.
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.
Police closed Bridge Road in Leavesden at the junction of Watford Road, known locally as Hunton Bridge, just before 10.30am today to assist Hertfordshire Fire and Rescue Service.
A police spokesman said: "Local routes will be affected but Hertfordshire County Council’s highways team will be dealing with closures and any diversions."