More than 50 firefighters were scrambled to the fire in the Grafton Street apartment block yesterday at 3.52pm.
Residents described to The News the ‘petrifying’ scenes during their evacuation as the blaze tore across the top of the four-storey building.
No one was injured or required treatment, and no one was in the flat at the time.
Now police are appealing for witnesses as the initial enquiries suggest the fire was not an accident.
Det Con Hollie Loader said: ‘Our initial enquiries have led us to believe this fire was deliberately started, and officers will remain in Grafton Street today as we continue to investigate.
‘Fortunately, nobody was injured and the flat in question was empty at the time.
‘We are keen to speak with anyone who may have any information about the fire, or who saw or heard anything suspicious yesterday afternoon.
‘If you can assist, please call 101, quoting the reference 44220186268. Alternatively, you can call Crimestoppers anonymously on 0800 555 111.’
Firefighters were sent to the scene from across Hampshire, including Southsea, Cosham, Portchester, Havant, Eastleigh, Fareham, Hayling Island, St Mary’s, Waterlooville, Beaulieu and Alton.