Steven Lewis has not been seen since he vanished from his residence at the Southsea Leisure Park, in Melville Road, on Friday, July 15.
The 40-year-old has been battling mental illness for the past four years, but recently his behaviour has become ‘increasingly worrying’, his family have said.
They are fearful for Mr Lewis’ safety – and that of the public if he is not found soon.
Sister-in-law Sarah-Jane Lewis, of Gosport, is desperate for Steven to return home.
‘Steven is not well and is really very paranoid,’ said the 43-year-old.
‘He thinks that everyone is against him and he thinks that he is being followed.
‘Most of all we just want him home safely without hurting anyone.
‘He has not had his medication for quite a while.
‘The longer he goes without this, unsupervised, the more he will deteriorate.’
This is the first time Mr Lewis has gone missing, his family say.
The police are aware of the situation and have been attempting to find the former fabrication worker.
Mr Lewis, who grew up in Milton, is believed to still be somewhere in Portsmouth.
He has a silver Renault Clio, registration GU07 ZJK, although police have told the family that none of the city’s CCTV cameras have seen the vehicle leaving Portsmouth.
Sarah-Jane added: ‘The biggest impact is on his mum.
‘She is 70 and is bed-ridden and has been in hospital for most of this year.
‘Him missing has had a massive impact on her.
‘We just want him to return to us safely.’
Mr Lewis does not have any close friends in the city, his family added.
Sarah-Jane said despite all their efforts, there had been ‘no trace of him’ since he vanished 12 days ago.
She said her brother-in-law had a ‘big interest in gambling’ and might stop at local bookmakers.
But she added: ‘Police have been checking for him, but they haven’t had any luck so far.’
In a final effort to be reunited with Mr Lewis, the family resorted to placing an appeal over social media – but this has not turned up any information about Mr Lewis’s whereabouts.
Those who see the 40-year-old are urged to call police on 101, quoting reference number 4416 0269223.