Witnesses heard “explosions” on the set before the blaze engulfed the studio also used to film James Bond on Tuesday.
There are no reports of any injuries, so far.
An anonymous source told The Sun: “A set made up of a lot of wood, and trees etc was currently being built in the Richard Attenborough stage.
“Rumour is a piece of set has caught alight and spread across the rest of the set. Six fire engines from local stations have arrived and currently tackling the fire.
“Some crew in the studio were shaken but evacuated without injury apparently.”
A Buckinghamshire Fire and Rescue Service statement said 12 crews had been called out to the blaze at 3.15pm.
The firefighters used ten sets of breathing apparatus, three hose reels and one main jet to tackle the fire.
Thames Valley Police and South Central Ambulance Service were also in attendance, supporting the multi-agency response to the incident.
Production on the upcoming Snow White and the Seven Dwarfs film began on March 1.
Filming is scheduled to end on July 8. The film is expected to be released sometime in 2023.
Rachel Zegler was cast as Snow White and Gal Gadot was cast as The Evil Queen.
In response to criticism from actor Peter Dinklage over the “stereotypical portrayal of dwarfs”, Disney announced the film will replace the titular characters with “magical creatures” and casting for them will begin shortly.
There have been several fires at Pinewood over the years, with the set burning down during the filming of Ridley Scott’s Legend in 1984.
Another fire broke out after production ended on the James Bond film Casino Royale in 2006.