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A fire broke out at the former St Bartholomew's Hospital in Rochester which was Britain's oldest surviving hospital before it closed about three years ago.
Fire crews from four stations attended after reports of thick black acrid smoke coming from the building at about 6.15pm yesterday.
Police have confirmed they are investigating the incident and inquiries are ongoing.
The blaze is believed to have been sparked by a camp fire ignited on a flat roof of the three-storey premises in New Road.
Crew manager Zac Taylor said access proved difficult to the highly secured listed hospital which dates back to 1861 with the initial building founded on site in 1078.
Mr Taylor based at the Rochester station said: "We initially went in around the back but then had to go back on to New Road in the front with our equipment.
"It looked worse than it was originally because of the smoke.
"But in the end only two appliances were needed."
The density of the smoke was caused by burning tar and foam insulation on the roof of a flat on the ground floor
It took firefighters about 20 minutes to damp down before leaving at about 7pm.
St Bart's has been sold for re-development after shutting in September 2016.
In a prime site overlooking the river and Victoria Gardens it is ear marked for luxury apartments. But parts of the historic building, such as the mortuary which is listed, and stones and plaques will be retained.
It is situated in a conservation zone and any development will need to preserve and enhance the character of the area.