The fire in the disused building at Bedford Hospital started just after 6pm on Thursday. Some 60 firefighters worked through the night to get the blaze under control.
The inferno ripped through the boarded-up building sending plumes of thick smoke into the air.
Superintendent Jim Lunn, from Bedforshire Police, said the incident is being treated as deliberate, urging anyone with information to come forward.
He said: "We take all reports of arson very seriously, particularly as the fire was so close to operational hospital buildings, and we are continuing to investigate the circumstances of this incident.
“At the moment we are treating this as deliberate.
The fire in the disused building at Bedford Hospital started just after 6pm on Thursday (Image: SWNS)
Some 60 firefighters worked through the night to get the blaze under
