Six-times champion jockey Kieren Fallon was involved in a plot in which he and two other jockeys agreed to cheat in 27 races to make horses lose, the Old Bailey was told today.
The prosecution alleged there was an agreement not to permit horses to " run on their merits" and that "riding practices would be used if necessary" to interfere with their running.
The horses did not always lose as they were meant to - but when they did, it allowed a dishonest syndicate run by businessman Miles Rodgers to make money on the online betting exchange Betfair, the jury was told.
Fallon, who rode Dylan Thomas to victory in the Prix de l'Arc de Triomphe in France yesterday, stood in the dock to hear the charge read to the jury this morning.
The six defendants, including jockeys Fergal Lynch and Darren Williams, deny being part of a race-fixing scam.
Fallon, 42, formerly of Newmarket, Cambridgeshire, but now of Tipperary, Ireland, Fergal Lynch, 29, of Boroughbridge, North Yorkshire, and Darren Williams, 29, of Leyburn, North Yorkshire, deny the charges.
Lynch's brother Shaun Lynch, 37, of Belfast, former racing syndicate director Miles Rodgers, 38, of Silkstone, South Yorkshire, and Philip Sherkle, 42, of Tamworth, Staffordshire, have also pleaded not guilty.
They are charged with conspiracy to defraud between December 2002 and September 2004 by interfering with the running of horses to ensure they lost races, defrauding Betfair punters and others putting money on the races.
Rodgers is also accused of concealing the proceeds of crime.
Jonathan Caplan QC, prosecuting, told the court: "This case concerns a serious allegation of fraud.
"It is unusual because it is also concerned with sport and any allegation of fraud in that context obviously undermines the integrity of the sport in question."
He said the 27 races were run on various dates and in different parts of Britain.
Mr Caplan said: "The prosecution case is that there was an unlawful agreement or conspiracy between these defendants and other persons that those races should be fixed.
"The defendants in this case did not fix races to ensure a particular horse won.
"On the contrary, they fixed the races to ensure that the horses in question lost.
"The object of the conspiracy was to wager large amounts of money on a particular horse to lose in each of those races whilst knowing that the jockey was prepared, if necessary, to cheat by stopping the horse."
Mr Caplan told the court that the betting was organised and conducted by Rodgers, who had numerous accounts in different names with Betfair.
"He was the organiser of this conspiracy and was the one who was most involved," said Mr Caplan.
"On race days, Rodgers had direct contact by mobile telephone with Fergal Lynch and Darren Williams.
"Kieren Fallon was more cautious and Rodgers had indirect contact with Fallon using an intermediary, Shaun Lynch, to a lesser extent Fergal Lynch, and latterly, Philip Sherkle."
Mr Caplan said the jury would be invited to look at all the circumstances of the case and "at the pattern which we say clearly emerges from them".
He said that "there was a criminal conspiracy in operation to fix numerous horse races in this country to the detriment of the betting public, and that this conspiracy was only brought to a close by the arrests of these defendants".