A barn was set on fire in Chatteris late on Monday night.
Crews were called to the blaze in Longwood Fen Drove, Chatteris at 10.40pm.
Rubbish and tyres were on fire inside a barn, measuring 70m by 20m.
The cause of the fire is believed to deliberate. Anyone with information should contact the police on 101 or the independent charity Crimestoppers anonymously on 0800 555111.