A few days before the 1999 Liège-Bastogne-Liège, one of cycling’s most prestigious races, which takes competitors on a 164-mile loop around the cultural hub of Belgium’s Wallonia region, Frank Vandenbroucke named the exact point where he was going to make his winning move, down to the very house number. Even for the pre-race favourite, it seemed like pure madness: there are umpteen variable factors to confront in this sport, not least 150 rivals keen to stop him. Such a pronouncement was like specifying the minute in the cup final when the winning goal would be scored, or the actual punch and round when the boxing knockout would come. Muhammad Ali, eat your heart out.

The Belgian was so confident and so good that he didn’t need the element of surprise. Racing through the lush Ardennes, he ditched his rivals 300 metres before the promised address with characteristic style to win the race. While other riders pushed the pedals in an ungainly laboured manner, he turned them in a languid circle that belied the steepest gradients. Beauty on a bike personified. After years of Vandenbroucke showing princely promise, international cycling appeared to have its new king. And that evening, he had a right royal time, celebrating in the bar where he grew up in the nondescript Franco-Belgian border town of Ploegsteert. The party raged on for three days.

There’s a belief that Belgians are unremarkable, hence the old party game of racking your brains to think of 10 famous ones. When it comes to cycling though, they have the greatest in history – Eddy Merckx, a five-time Tour de France champion, who won over 400 races in the 1960s and 1970s. Vandenbroucke, just 24 years old when he won Liège-Bastogne-Liège, seemed to be his successor. And the legend himself approved. ‘With his talent, Frank is the Johan Cruyff of cycling. He could win anything. Except maybe the Tour de France,’ Merckx told the press.

He blew all his rivals away in the charisma stakes too. He often won races in dazzling breakaway fashion; he bleached his hair and wore a chain, like a cycling version of Eminem, and gave thrilling quotes to the press, who dubbed him the ‘golden boy’. His fans went further, nicknaming him simply ‘God’.

The acclaim of the Belgian public was like a drug for Vandenbroucke. Heads turned whenever he walked into a restaurant; he’d buy sports cars, and expensive suits which he wore once then abandoned at the dry cleaner’s.

‘It was like being a king on a throne. Everywhere I went I was VDB the super-talent, the young god,’ Vandenbroucke wrote in his autobiography Ik Ben God Niet (I Am Not God). ‘People started to stutter when they spoke to me. How long does it take before you begin to believe the hype yourself?’

Nobody could know the full extent of the ugliness behind the era’s eye-catching performances. The 1990s were some of the most heinous years for weapons-grade doping in cycling, with a panoply of physiology-changing drugs, such as anabolic steroids and human growth hormones, circulating in the peloton. Erythropoietin (EPO) was the game changer, a hormone that stimulates red-blood-cell production, thus increasing the amount of oxygen that the blood can carry to the muscles. As the drug raised an individual’s haematocrit – the proportion of blood that consists of red blood cells – the sport’s governing body, the Union Cycliste Internationale (UCI) set a limit of 50 per cent in its tests to detect manipulation.

Rather than preventing cheating, it ensured riders could dope as long as they didn’t breach that limit. ‘It gave the cyclists a target,’ Australian stem-cell scientist and anti-doping expert Robin Parisotto says. ‘Most endurance athletes have a haematocrit of around 40 to 44. So there’s huge scope for improvement, and yet still stay under the legal detection limit. If your haematocrit is 45, you could actually up that by five units. You’re looking at around a 15 per cent improvement in blood-oxygen-carrying capacity before you’re even flagged.’

Those with a smaller natural haematocrit had an advantage, physiological room to manoeuvre – and Vandenbroucke claimed his was 42. In a 2002 report based on a leaked court dossier that appeared in Flemish newspaper De Morgen, Vandenbroucke admitted to taking EPO and growth hormones: ‘Modern cycling simply is not possible without these kinds of products . . . without that, I would not have been able to win.’

Cycling’s name was dirt after the seismic events of the 1998 Tour de France. A team helper from the French Festina team was intercepted pre-race with a small pharmacy’s worth of banned drugs in his car, leading to the arrests of top athletes, multiple team withdrawals and only half the original field making it to the finish. The scandal nearly stopped the race, but it didn’t appear to stem the cheating. ‘A large proportion of the peloton were participating in blood doping,’ Parisotto says of that time. By the late 1990s, numerous teams had their own internal systems, with doctors administering banned products.

There was even more dysfunction behind the scenes in Cofidis, Vandenbroucke’s squad. In the winter of 1998, his teammate Philippe Gaumont introduced him and other cyclists to the euphoric high that resulted when Stilnoct sleeping pills and alcohol were mixed. It became a ghoulish game: as resistance to the medication grew, they took tablets into the double figures, eventually succumbing to the sedative effects, unable to recollect their wild nocturnal behaviour. The mix could leave them stoned or jumping up and down on mattresses and screaming like banshees. Yet Vandenbroucke could stay up partying half the night, and still beat his cycling rivals by day.

Within three weeks of his sporting triumph on the hills outside Liège, Vandenbroucke was living a waking nightmare, and it had nothing to do with the sleeping pills. On 9 May, he was questioned at the Quai des Orfèvres in Paris, France’s equivalent of Scotland Yard, a place more accustomed to holding terrorists and serial killers than leading athletes. Vandenbroucke had been brought in as part of a police operation aimed at his guru, Bernard Sainz (often referred to as ‘Dr Mabuse’), and lawyer Bertrand Lavelot, suspected of being at the head of a supply network of illegal substances, including EPO, anabolic steroids and corticosteroids.

Sainz exerted a great advisory influence over Vandenbroucke. A homeopath and unlicensed doctor without an official medical degree, who had worked in cycling and horse racing for decades, he has since been jailed on doping-related charges and is now under house arrest.

Fourteen cyclists were apprehended, including his teammate Gaumont, yet Vandenbroucke was the one in the eye of the media storm. He wasn’t charged, but his Cofidis team suspended him nonetheless. The glances at him in the street were no longer admiring.

‘Even if he didn’t have a positive test, there was a mystery he was attached to. He came back, he went training but it always stayed a little bit inside him,’ his childhood friend and former teammate Steve De Wolf says. ‘He felt hunted. He wasn’t used to people trying to bring him down.’

The case broke something inside Vandenbroucke. Stewing at the perceived injustice, he took amphetamines and sleeping pills heavily. That winter, he barely rode his bike. By the end of 2001, now aged 27, he had gone from the sports pages to the tabloids, from winning the biggest races to not being in a fit state to even start them.

His wife Sarah Pinacci, an Italian model he romanced at a leading bike race by promising (accurately) that he would win it for her, sought out psychologist Jef Brouwers, who tried to help Vandenbroucke by examining the circumstances that had taken him out of the winning process.

Many of the answers lay in his past. As a four-year-old, Vandenbroucke had fractured his femur after being hit by a rally car while on a bike ride with his family, leading to a shortened left leg. Ever since that moment, he was doted on by his bar-owner parents, Jean-Jacques and Chantal, who had been afraid of losing him. And when this cycling-obsessed family realised the teenager’s incandescent talent, they gave him everything he needed to focus on the sport. In contrast, his older sister Sandra worked as a weekend waitress at the family’s hostelry.

‘He was a spoiled boy. For me, he was terrorising his environment,’ Brouwers says. His lifestyle of fast cars, outlandish boasts and apparent arrogance was a façade. ‘Frank was shy and introverted, not the extrovert he appeared to be. He was continuously looking to be praised,’ Brouwers says. He felt that he needed to win to exist.

A condition of Brouwers’ collaboration with the cyclist was that he stop working with Sainz. But Vandenbroucke lied and continued to see the fake doctor – and it came back to bite him. In 2002, Sainz was caught speeding in an uninsured car, having come from Vandenbroucke’s house. A subsequent police search found a box of doping products there, which included EPO, morphine, testosterone and corticosteroids. Vandenbroucke’s subsequent ban from cycling lasted four months – effectively losing him another season in his prime.

However, the biggest damage was to his image. During the subsequent trial in the Belgian city of Dendermonde on 28 February 2002, Vandenbroucke was shown handcuffed to a policeman on the way to see the judge. The pictures of a manacled champion went round the world. ‘He always talked about that moment he was on TV, handcuffed, shown as a criminal,’ Brouwers says. ‘It was a media show and Frank was a lovely guy, a nice guy. He was absolutely not a criminal. He was ill. He was ill. He was not a sportsman, he was a patient.’

Broken again, ‘VDB’ built himself up for a last hurrah at the 2003 Tour of Flanders, finishing a close second in Belgium’s most prestigious race. Having started the year ranked 575th in the world, it was an astonishing moral victory, after everything he had been through. ‘People always love comeback stories. Because they recognise that in life it’s all about falling down and standing up,’ De Wolf says.

But it wasn’t enough for Vandenbroucke. He went to his hotel post-race and sat in the bath for hours, silent and crestfallen. ‘He had a need for that applause,’ adds Brouwers. ‘And he got applause for his second place but he hated that. He didn’t want that, it’s a pity applause – an appreciation for the rider but not for the champion. I think that if he had won that race, it would have been another story.’ Instead, his fall from grace began in earnest.

Brouwers says that Vandenbroucke was a perfectionist and a genius, who was within the highest percentiles for verbal and non-verbal intelligence in his tests. But his actions in the mid-noughties did not always reflect this.

Some are stranger than fiction. There was the time he fired a rifle while on the phone to his wife after an argument, faking suicide, which led to a stand-off with an armed Belgian SWAT team. The time he stayed at the house of a Ghent nightclub promoter for six weeks rather than going home. The time he infiltrated an Italian amateur race, using the fake name Francesco del Ponte with a photo of one of the sport’s incumbent stars, Tom Boonen, attached to his racing licence.

There were dark moments. Vandenbroucke made two suicide bids, in 2004 and 2007, and was sectioned in a psychiatric hospital, weeks after Sarah told him that she wanted a divorce. She later told the Gazzetta dello Sport that he had systematically used amphetamines and cocaine and claimed he physically beat her several times, which Vandenbroucke denied.

Through all his mistakes, he stubbornly returned to cycling. It was his form of deliverance, even as the contracts grew smaller, with less prestigious teams and he failed to challenge in races. Asked about his regrets in 2007, Vandenbroucke said, ‘Being too lazy and not making sacrifices when I should have. Up to a certain point, I did it my way and took decisions alone. They said I was arrogant. But when I won, it was beautiful, à la James Dean. Maybe my ambitions were too big.’

Though he won over 40 professional races in his career, it wasn’t so much what he achieved or the promise he squandered, what mattered was how he made people feel. He was a sporting folk hero akin to footballers Paul Gascoigne and George Best, as talented and likeable as he was flawed, a bon vivant who talked to everyone, whose friends ranged from Michelin restaurateurs to blue-collar workers. No wonder he was irresistible to the media.

‘He was alone, surrounded by a crowd,’ Brouwers says. ‘He always felt abandoned by everyone, even if they made a fuss over him. For me, that was his drama. He was loved by so many people, but he was unhappy with himself all the time.’

In October 2009, he went on holiday to Senegal with friend and fellow professional cyclist Fabio Polazzi. On the first night, they met a sex worker called Seynabou Diop and went to a bar. Around midnight, Polazzi told Vandenbroucke that he was going back to their hotel.

Vandenbroucke was found dead at three o’clock the next afternoon at the Maison Bleue hotel. He had gone there with Diop, who last saw him asleep in their bedroom, with vomit at his feet. She claimed he was drunk, which Polazzi denies. ‘He was really tranquil and normal. I’d never have thought or imagined that it was the last time that I’d see him,’ Polazzi says.

The official verdict was a double pulmonary embolism and it was registered as a death from ‘natural causes’. There was little that seemed natural about it; even someone with a lengthy history of addiction and substance abuse does not drop dead from natural causes at the age of 34.

The local gendarmerie found Stilnoct tablets, Xanax and insulin on his bedside table. The initial autopsy suggested puncture wounds in his left arm; suicide has to be considered as a possibility, given the presence of insulin, which he had used for past suicide bids. Moreover, by October 2009, he had little left: divorced, without a contract from a professional cycling team and running out of money.

Yet, Vandenbroucke was a master dissembler and concealed his pain from friends and family, who had thought he was in a far happier place. Many of those close to him talked of an accidental poisoning by Diop, who later admitted to stealing his phone.

The Vandenbroucke family refused a toxicological report. ‘I didn’t want one done. I don’t get Frank back with that. He was in the wrong place at the wrong time,’ his mother Chantal told Het Nieuwsblad in 2010.

Belgium mourned its fallen star with much fanfare; one estimate put the total number of people attending his funeral at 14,000. On top of such sporting grace, his mysterious end has added to his hallowed status. ‘Which proves for me his huge intelligence,’ Brouwers says. ‘Nobody can say he committed suicide, nobody can say he didn’t. The situation is unclear. There was a prostitute and needles, and you never know what happened. Was he killed? We can have about a hundred interpretations of this situation,’ he adds.

Only God knows.

God Is Dead: The Rise and Fall of Frank Vandenbroucke, Cycling’s Great Wasted Talent, by Andy McGrath (Bantam, £18.99), is out on Thursday. Order your copy from the Telegraph Bookshop.