Because Irish jockey Kieran Fallon is a defendant in the race-fixing "trial of the century" currently being heard at the Old Bailey in London. Fallon won a memorable Prix de l'Arc de Triomphe at Longchamp in Paris last Sunday, but was brought back down to earth with a bang on Monday as his fight against race-fixing charges began.

He described Sunday as "probably one of the best days of my life" after the Aidan O'Brien-trained Dylan Thomas won Europe's most coveted race. The four-year-old winner is owned by the Coolmore Stud syndicate. Fallon is their retained jockey and the group have resolutely stood by their man through the current race-fixing controversy.

Five co-defendants also face corruption charges along with Fallon after a City of London police investigation into more than 80 races between December 2002 and September 2004. "Although Kieren Fallon is just one of six defendants, it is not surprising that virtually all the media attention has been focused on him, " says Mark Costello, deputy editor of the Irish Field. "When he won the Prix de l'Arc de Triomphe on Dylan Thomas last weekend, the news was carried on the front page of all the broadsheets. Would this have happened if the court case was not ongoing?"

What's the prosecution's case?

The 42-year-old Clare-born jockey was accused on the trial's opening day of being part of a plot to throw races to make money for a crooked betting syndicate. Fallon, formerly of Newmarket, Cambridgeshire but now of Tipperary, Fergal Lynch (29) of Boroughbridge, North Yorkshire, and Darren Williams (29) of Leyburn, North Yorkshire, deny the charges.

Shaun Lynch (37) of Belfast, former racing syndicate director Miles Rodgers (38) of Silkstone, South Yorkshire and Philip Sherkle (42) of Tamworth, Staffordshire also pleaded not guilty. It is alleged they were involved in a conspiracy to defraud Betfair customers and others over six years.

Businessman Rodgers bet �2.12m on various internet accounts that 27 horses, ridden by Fallon and two other jockeys, would lose. Before each race, the jockeys agreed to lose the races, claimed Jonathan Caplan QC, prosecuting. Seventeen of the horses were ridden by Fallon, the six-time champion jockey. But he won five of the races, losing the syndicate around half a million pounds, according to the prosecution.

That Fallon won races he was supposedly committed to throwing will be the crux of the case for the defence.

The wins included one on the queen's horse, Daring Aim. This was said to have cost the crooks �138,000 the court heard.

The day after the race at Newmarket on 23 July 2004 there was an exchange of text messages between Fallon and Philip Sherkle, said to be the go-between.

Fallon, the court heard, texted: "They will take my licences off me if they drift like that last night. They are watching me." On 27 July, Sherkle wrote: "I am away weds morning just text as normal make sure u do please I don't want any grief from them when I get back thanks."

An undated message from Fallon to Sherkle read: "no I can't chance it". Angered by Fallon's wins, Rodgers . . . who was thought to be working for a group of backers in Spain . . . tried to confront him, the court heard. But during August 2004 Fallon sought to find a method of squaring the loss that he had caused.

The prosecution accept that there is no evidence that Fallon ever received any money or benefitted from Rodgers or anyone else connected with the so-called conspiracy, but allege he was nonetheless involved.

What is the significance of the texts?

If the Joe O'Reilly murder trial taught us anything it was to never, ever text anything that can later be used against you in a court of law. Unlike phone calls, which are only recorded if a phone is tapped, a record exists of every text message sent for a couple of years. The case is based partly on bugged conversations from Rodgers' Mercedes car and the carpark of his Italian restaurant with Fergal Lynch, Philip Sherkle and others.

There were also text messages found on mobile phones and a pattern of phone calls and betting on the online betting exchange, Betfair. This has added much colour to the case and captured the general public's imagination.

What's been the most damning evidence heard against Fallon?

That the champion jockey appeared to stop riding Ballinger Ridge as he came into the home stretch with a huge lead in a race in March 2004. Fallon came second in the race which he should have won "at a canter".

Fallon later told a stewards' inquiry that he had given the horse "a breather" and was afraid he would become "legless" if he kept up the pace. The Jockey Club had tipped off officials at Lingfield racecourse before the race that a large amount of money had been placed on the horse to lose. The sport's ruling body later found that Fallon had made an error of judgment in the race.

What case has defence counsel put forward?

That Fallon was the central player in the sophisticated, multi-million pound scam is "absurd" and "simply ridiculous, " according to John Kelsey-Fry QC.

He said the "extraordinary case" relied on "snippets" of bad-quality probe recordings, average evidence and poor police interpretations that were taken "out of context".

The defence rejected prosecution claims that Fallon "accidentally won" in five of the races. It was also denied that there were "corrupt" relationships between several of the other defendants and Fallon, who spoke often as if they were friends. KelseyFry added that it was only natural for a professional gambler like Rodgers to seek the opinions of a champion jockey for "tips".

Will this case adversely affect racing here?

Probably not. "It is worth noting that none of the races under suspicion were run in Ireland, so there is no reason why the sport here should be adversely affected, whatever the outcome, " says Mark Costello of the Irish Field. Antrim-born showjumper Jessica Kurten is currently challenging a positive doping test on her horse Castleforbes Maike. Despite this, equestrian sport in Ireland is still considered top-class. The case against Fallon is expected to last several months.