Everything changes as Take That relight the fire (but they're not back for good)

At the height of Take That's popularity in the early 1990s, Robbie Williams was a 19-year-old alcoholic who would down at least one bottle of vodka a day on his own. "I would drink myself to oblivion every night by myself," Williams reveals in a 90-minute ITV 1 documentary, Take That... The Final Report due to be screened at 9pm on Wednesday, "I was so lonely." When he left the band in 1994 after a lost weekend hanging out with Oasis at the Glastonbury Festival, Williams says: "I had physically and mentally gone."

The rest of the boy band had no idea that their youngest member, the joker with a pronounced exhibitionist streak, had a drink problem. "He hid it well," Jason Orange says in the film, which contains candid revelations about the less wholesome aspects of being a teen idol.

Drugs were apparently rife in the Take That camp. One of them refers casually at one point to "acid, speed, weed" although cocaine, the drug to which Williams became addicted early in his solo career, is not mentioned.

Previously unseen home video footage, shot by their manager, shows the band backstage, often surrounded by sexually available young female fans. During one impromptu after-show party in the dressing room, he simulates masturbation with a microphone stand and stages a mock ejaculation. We learn that Take That's management encouraged the boys to engage in all the casual sex they wanted but outlawed regular girlfriends.

But contrary to the perception of the young Robbie as an insatiable teenage love machine, pleasuring groupies all night long, by his own account Williams would retreat to his hotel room alone most nights to commune with the bottle.

The Final Report gives a unique insight into the formative years of Britain's most popular entertainer. The picture that emerges is of an artist for whom stardom came suddenly, and at such a young age - Williams was just 17 when girls started mobbing Take That - that fame and personal happiness soon became mutually exclusive. He has never enjoyed a stable long-term relationship, has spent time in rehab and therapy and now lives on his own in Hollywood.

"I would swap everything I have for what he's got," Williams says of the domestic contentment enjoyed by his former Take That partner Gary Barlow, who is now married with a daughter and living quietly in Cheshire.

The rivalry between the pair, who embarked on solo careers after the group split in 1995, appears to have abated. Williams has been vastly more successful than Barlow - the most musically accomplished member and the one widely tipped to follow in the footsteps of George Michael after the split - and is magnanimous now about Barlow's songwriting, which he used to rubbish. He talks of the "guilt" he feels about the feud and doubts that the feeling will ever go away.

Neither, it seems, will the continuing hatred he feels for Take That's manager, Nigel Martin Smith. Martin Smith has often stated publicly that he only hired Williams in the first place as an insurance policy in case one of the others dropped out. He is widely believed to have engineered Williams's exit from the group after the Glastonbury incident. He then sued his former charge for £200,000 which he claimed to be unpaid managerial commission. In the film, Martin Smith says of Williams "I haven't got a nice thing to say about him."

The feeling is mutual. Williams calls Martin Smith "the third most disturbed person I have ever met" and while the manager claims the credit for saving him from a humdrum life in Stoke-on-Trent, Williams insists on screen that he would have become famous without Take That.

Take That were the most successful British boy band since the Beatles, selling 25 million albums. Although only Barlow wrote any of the music, each member is believed to have walked away with about £5m.

The only former bandmate Williams had seen since the split was Mark Owen, who now lives in the Lake District. Jason Orange spends most of his time on Ibiza and suffers problems sleeping. Howard Donald lives in Bournemouth with his daughter. Like Barlow, they saw nothing of Williams after he walked out of their rehearsal room in July 1994, leaving Take That to soldier on as a quartet.

The finale of The Final Report features a staged reunion at Cliveden country house hotel in Buckinghamshire, filmed last month, in which Take That reconvene to mark the 10th anniversary of their break-up and the release of an album of their greatest hits.