At 6.31pm on Monday (11) crews were called to a fire on Back Lane in Cambourne.
Crews from Papworth, Cambridge and Gamlingay attended the scene.
Firefighters arrived to find a well-developed fire involving a portable building. Wearing breathing apparatus they extinguished the fire using a jet and returned to their stations by 8.30pm.
The cause of the fire was deliberate. Anyone with information should contact police on 101 or Crimestoppers on 0800 555111.