Patients at Southampton General Hospital had to be evacuated to other areas of the building after fire broke out.
Hampshire and Isle of Wight Fire and Rescue Service was called to Southampton General Hospital at 5.30am.
Nobody has been injured and the fire has now been contained.
Roads surrounding the hospital have been closed by emergency services.
A spokesperson for University Hospital Southampton said: “Visitors are asked to stay away and all planned outpatient appointments are cancelled today.
“Parking is being made available on site for staff who are coming into work.
“Our Emergency Department is currently diverting patients away unless your condition is deemed life or limb threatening.
“We are asking anyone requiring medical attention to use other services in the community including the Urgent Treatment Centre at RSH or Lymington.”
The fire service said: “Crews from across Hampshire were called to reports of a fire at University Hospital Southampton at 5.30am this morning.
“At its peak, more than 110 firefighters were mobilised to the incident.
“Breathing apparatus, hose reel jets and an aerial ladder platform are in use.
“The fire is now under control, crews remain on scene to dampen down.
“Nobody has been injured as a result of the fire and patients in all affected areas have been evacuated to safe areas of the hospital.”
In a post on Facebook, Southampton Test Labour MP Satvir Kaur said: “Huge thanks to our incredible emergency services for containing the situation.
“You are all incredible.”