Racing: Jockey Club smells a rat as 280-1 shot's victory costs bookies £500,000

To the casual gambler, Exponential might not have appeared a great bet as he lined up at Nottingham racecourse on Monday. He was wearing an eyeshield and blinkers and in his only other race had finished last - by 17 lengths.

To the casual gambler, Exponential might not have appeared a great bet as he lined up at Nottingham racecourse on Monday. He was wearing an eyeshield and blinkers and in his only other race had finished last - by 17 lengths.

But by the time the two-year-old gelding had galloped past the winning post at Nottingham a metre ahead of his closest rival, alarm bells were ringing in betting shops across the land.

The victory of the 100-1 shot in the hitherto little-known Brothers Maiden Auction Stakes was yesterday estimated to have cost bookies more than £500,000.

The Jockey Club, horseracing's governing body, said it was investigating the circumstances of the win as part of an agreement with the betting industry to scrutinise dramatic improvements in the form of a horse.

John Maxse, a spokesman for the Jockey Club, said: "This was a horse which had performed poorly in its only other race. Our security department is looking into it just to make sure there was nothing untoward."

Rapid improvements in the form of horses are far from unknown in racing but after recent scandals involving tipping by senior jockeys, the authorities are keen to be seen to investigate the most dramatic victories. Prior to the race, Exponential, which is owned by an anonymous syndicate called the Exponential Partnership, had been priced at 280-1 on betting exchanges.

The odds shortened to 100-1 once betting shops opened on Monday morning and bookmakers found themselves inundated with substantial bets for the horse, which is based in Newmarket. The eventual starting price was 8-1.

Ladbrokes, said it had received four bets of £100 each way at 100-1 while Coral said it had several bets of £200 at the same odds, winning each punter £25,000. William Hill put its losses at £100,000.

The victory of Exponential, which was taking part in a race contested between novice and largely-untested runners, was immediately examined by stewards at the racecourse.

The officials accepted the explanations of the trainer, Stuart Williams, that the horse had benefited from the improvement of its first race, six weeks ago, and gained in strength during the intervening period. They also ordered the horse to undergo routine dope testing.

Mr Williams said the large wins were down to the shrewd judgement of a small number of gamblers. He said: "I think it was a question of some punters spotting good value for money."

The Jockey Club also played down suggestions that the £500,000 bill to the betting industry for Exponential's victory was the result of a "sting". Experts said that it was more likely that after people close to the horse had expressed their confidence in its form by placing large bets, word had spread among the betting fraternity of a potential big winner.

Mr Maxse said: "At the moment there is no suggestion of foul play and certainly no evidence of an orchestrated betting fraud."