It said the incident happened “right at the start of the game”, with the referee taking the players off the pitch “to allow for treatment to be administered”.
“Despite the valiant attempts of the emergency services, the supporter unfortunately passed away,” the statement read.
“Next of kin are aware. Our sincere thoughts and condolences go out to the supporter’s family and loved ones at this sad time.”
The club said it was grateful to all the Portsmouth and Middlesbrough fans inside the stadium “for the compassion, patience and understanding they showed in such distressing circumstances”.
