Police have warned farmers to look out for people or vehicles near hay stacks and barns after a series of fires.
Kent Police said a fire in a barn near Westerham, in Kent, on Thursday was being linked to other incidents in both Sussex and Surrey.
Firefighters were called to the Westerham fire at 2000 BST and remained there into the next day.
Detective Sergeant Andy Nicoll, who is investigating the fires for Kent Police, said hay barns might appear to be an easy target but the potential danger to the public was great.
'Wanton stupidity'
He urged farmers and landowners to watch out for suspicious people or vehicles near barns.
Mr Nicoll said: "I am sure the person responsible thinks they are just setting light to straw but what they have to realise is other items may be stored there.
"At Westerham there were gas cylinders nearby and such a wanton act of stupidity could have resulted in a terrible tragedy."
Anyone with information is urged to contact Kent, Sussex or Surrey Police.