Arsonists targeted a wooden lean-to shelter on Friday, February 15, damaging bales of hay stored inside.
The incident happened at Legge-Bourke Court, Wisbech, between 5pm and 5.50pm.
Firefighters do not know what started the fire in the lean-to, which was next to a brick built building. The shelter was completely destroyed.
Anyone with information is asked to call police on 101 or Crimestoppers, anonymously, on 0800-555111.