Arsonists set fire to a derelict building at a former RAF base.
Firefighters from Ramsey and Huntingdon were called to RAF Upwood at about 12.45am on Sunday.
A spokesman for Cambridgeshire Fire and Rescue Service said: “There was a fire in a derelict building measuring 10m by 10m used formerly as the old guard house.
“Firefighters wearing breathing apparatus used one jet and one hose reel to extinguish the fire, and positive pressure ventilation fans to clear smoke.”
Firefighters left to scene at 2.55am.
An investigation revealed that the cause of the fire was deliberate. Anyone with information should contact police on 101 or Crimestoppers, anonymously, on 0800 555111.