Remains of an unauthorised bonfire were discovered in the grounds of All Saints Church in Boyn Hill Road in Maidenhead on Saturday morning.
Church sacristan and warden Christine Pocock discovered the ashes and charred ground by the north porch of the church's outer wall at about 9.30am.
The flames was fuelled by the cardboard removed from nearby recycling bins.
It is believed that the bonfire was ignited late on Friday evening and burnt for a considerable time throughout the night before going out by itself.
The fire service were informed but not required to attend the scene as only minor damage was caused to a lightning conductor that runs down the porch wall.
The church believe the fire was started by a homeless person who was trying to keep warm.
Christine, who has never come across a situation of this nature before, said: "I am very sad that somebody had to do that because they were so cold."
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