In a statement, Hampshire and Isle of Wight Constabulary said: “Sadly, despite the very best efforts of the emergency services, a 25-year-old woman was pronounced deceased at the scene. Her next of kin have been notified.
“A man in his 60s, who was also inside the property at the time, has been taken to hospital and is in a life-threatening condition.
“We would ask that people respect the family’s privacy at this unimaginably difficult time and refrain from any speculation.”
Police said officers would remain at the scene for the rest of Sunday.