THE family of missing student Jack O'Sullivan have made a formal complaint about Avon and Somerset Police force's attempts to find him.

Jack, 23, was last seen 100 days ago in the Hotwells area of Bristol as he walked home from a night out.

The last confirmed sighting of Jack was at 3.13am as he walked onto a grass area at the junction of Brunel Lock Way and Brunel Way.

Police say they have spoken to hundreds of cabbies in the area and reviewed thousands of hours of CCTV but Jack has still not been found.

His family say they have ''lost all faith'' in Avon and Somerset Police's investigation.

They have now put in a formal complaint about their experience, the BBC reports.

They say that errors were made in early searches and crucial CCTV footage was initially missed.

His mum Catherine said she found one sighting on CCTV herself after she was allowed to view some of the material.

She told the BBC: ''To have footage of my son in their possession the day after he was missing and for someone not to see that is just ludicrous.''

Data shows that his phone was still in use hours after he was last seen on CCTV.

His bother Ben told the BBC: "We've been told for weeks and weeks that there is the possibility of the National Crime Agency helping and providing their resources, but despite asking...there still has been no progress.''

An Avon and Somerset Police spokesperson said: "This investigation has included reviewing and re-reviewing more than 100 hours of CCTV footage, carrying out expert-led searches by land and water involving multiple teams, including the dog unit, drone unit and specialist dive team, proactively seeking and acting on advice from national policing specialists, and issuing multiple appeals to the public and media for information.

"Sadly, despite these efforts we’ve been unable to find Jack to date.

"We fully recognise the distress and anguish this has had on Jack’s family and our thoughts remain very much with them," they added

"We’re determined to do all that we possibly can to find the answers they so desperately need."