The group ' whose latest single 'Flood', their first as a five-piece since 1995 ' have been followed by cameras for the last 18 months in order to show fans a true account of their reconciliation.
A source close to the band said: 'Cameras have been with them right through the last chapter of their lives. The lads really wanted this to be an honest account of what happened when they were making friends all over again.'
Robbie left the group in 1995 following a rift with his bandmates Gary Barlow, Howard Donald, Jason Orange and Mark Owen, but the group are now said to be closer than ever.
Mark - who earlier this year confessed to cheating on wife Emma and spent a month in rehab to deal with his alcohol problems ' is said to have found the filming most difficult but the film shows how his bandmates supported him through his ordeal.
The source added to The Sun newspaper: 't wasn't easy and they've had to bare their souls more often than they had planned. Mark had a lot of problems in his private life when he admitted to his drink problems and marriage difficulties. But it just showed how tight they are as a unit. '
The film will be released in British cinemas in December.