The 14th-century thatched and timber building at St Bartholomew’s Priory, in Clermont Avenue, Sudbury, has been lost following the blaze on Tuesday night.
Police have now confirmed that the fire is being treated as arson and are urging anyone with any information to contact them.
The barn as well as the chapel and farmhouse at the site, have been on Babergh District Council’s Buildings at Risk register for a number of years due to the “vulnerability” of the unoccupied buildings to vandalism and other acts of destruction.
Members of the community have said they foresaw the disaster, and calls are being made for security to protect the remaining two buildings.