Arsonists set 300 bales of straw alight in Chatteris on Monday June 15.
Firefighters were called to the deliberate blaze on Doddington Road at 5.17pm.
One crew from the town and another from Ramsey were called to the stack fire.
When they arrived, the crews found approximately 300 bales of straw well alight.
The firefighters used hose reels and drags to break up the stack and get the fire under control.
Crews returned to their stations by 9.30pm before re-inspecting the incident overnight.
“The cause of the fire was deliberate,” said a Cambs Fire spokesman.
Anyone with information should contact police by visiting cambs.police.uk to submit an online report or call Crimestoppers on 0800 555111.
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