'I didn't notice it when I took the picture,' said Mr Rahman, 28, of London Road, North End. 'But then I looked a bit closer and zoomed in. It's a bit weird really, it's quite freaky.
'I've never really believed in UFOs. I'm not really sure what to make of this.'
Mr Rahman took the picture where the Tricorn Centre once stood, a site which is now a car park near Commercial Road. He took the photo just after midnight last Thursday.
Hilary Porter from the British Earth and Aerial Mysteries Society based in Farnborough, said Hampshire has a lot of UFO sightings and believes this one to be real.
She said: 'It would be very difficult to fake that photo and the UFO is at a tilt, which is the way they normally fly. You don't normally see that sort of UFO over this country, we generally get orange orbs, so the photographer who got this picture has got quite a coup.'
But others are more sceptical.
Professor Bob Nichol is one of the top astrophysicists from the University of Portsmouth's Institute of Cosmology and Gravitation.
He said: 'There's a very good chance that somewhere in the universe there are alien lifeforms. But sometimes my healthy scepticism takes over.

'This picture could be real but the question I ask is that if a big spaceship was to come down over Portsmouth, why would only one person get a picture of it? Surely with mobile phones, everyone would be snapping at it.'
The Ministry of Defence would not comment on individual sightings and could not check if there were any aircraft in the vicinity at the time.
An MoD spokesperson said: 'The MoD examines reports solely to establish whether UK airspace may have been compromised by hostile or unauthorised military activity.
'Unless there is evidence of a potential threat, there is no attempt to identify the nature of each sighting reported.'