Firefighters were called to the former Majestic Snooker Club in Kingston Road, North End at about 12.30am today.
Crew manager Simon Bone from Cosham fire station said: ‘It was well alight when we turned up. We don’t know how it started and fire investigators will be working on it. There are still some crews there making sure it doesn’t reignite.’
Southsea firefighters reported that there was thick black smoke in the area when they arrived, and the service sent out a warning to nearby residents to keep their windows closed overnight.
Firefighters said there had been a partial roof collapse but that the fire and been contained in one room of the derelict building.
A police spokesman said: ‘Road closures were put in place whilst emergency services dealt with the fire, with the road being reopened just after 5am.’
He added that at the moment the fire it is not being treated as suspicious so the fire service will be leading the investigation, and that five people were displaced from their homes at the time of the fire.