The farmer whose 300-tonne hay bale went up in flames on land near Stubbings in Maidenhead believes it must've been started "by a third party".
Giles Philp - who owns Town Farm in Bisham Village but also uses land close to neighbouring Pinkneys Green - does not know how the fire in Henley Road started in the early hours of Monday morning but says the hay was too dry to have set alight on its own.
Spontaneous combustion of hay typically occurs when it becomes too moist.
Firefighters from Maidenhead, Windsor and Reading were called put out the blaze in the field close to the A404 at about 3.30am.
Crews worked until 5.30pm on Tuesday to secure the scene.
Giles said: "300 tonnes of straw and every little bit has gone.
"I have to try and replace it before the winter comes in.
"It's got to have been started - the straw was dry.
"It started somehow by a third party, whether it was an accident or not I don't know," added the experienced farmer who says it might have been due to a discarded cigarette.
They hay is estimated to have been worth between £9000 and £12,000.
This is the fourth hay bale fire to have happened in Berkshire over the past two weeks following a blaze in Newbury on Monday, August 18, in Newbury again on Sunday and in Hungerford on Monday.
At 4am on Tuesday, 200 tonnes of hay burnt to the ground in Wantage, Oxfordshire and is being treated as arson.
Thames Valley Police is yet to investigate or confirm the cause of the fire in Maidenhead.
It has not been linked with the fires in the West of the county or Wantage.
Rural communities are being urged to be vigilant and farmers are being advised to stack hay in smaller piles if possible to avoid fire spreading.
Contact 101 if you have any information.
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