Racegoers at Southwell were left stunned when 125-1 shot Tioga Gold triumphed at Tuesday's meeting.
The eight-year-old took the handicap over a mile and six furlongs.
A distant last on his previous run, Tioga Gold gave trainer Lee James his first winner in just under six years.
The gelding, whose only other win came at Southwell in December 2001 when trained by Brian Meehan, was matched at 1,000-1 on the betting exchange Betfair to the tune of £40.
It means gamblers who staked the minimum wager of £2 on the horse won £2,000.
But conversely, traders who 'laid', or backed against the horse at that price, will have lost £2,000 for trying to secure a £2 profit.
Tioga Gold, ridden by Russell Kennemore, beat Nayodabayo (9-2) on the all-weather track by just under a length, with 11-4 favourite Bulberry Hill back in fourth.
He is the longest priced winner of an all-weather race in Britain.
Winning trainer James, who celebrated his 46th birthday on New Year's Day and leases boxes from Tim Fitzgerald at Norton Grange Stables, currently has four horses in his yard.
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She said he may run again in an amateur riders' race at Wolverhampton on Friday.
A £1 Tote exacta bet on the first two home in the eight-runner race paid a dividend of £870.10.
Bookmakers William Hill reported that one female internet customer was £625 richer after staking £5 on Tioga Gold to win.
Before his surprise win in the 1320 GMT race, the gelding had made the places a total of three times from 33 career starts.
In his last four contests, he had finished 11th (of 11 runners), eighth (of 12), pulled up and 19th (of 21).
Just 20 minutes after Tioga Gold won, Betfair punters were left reeling again with another shock result.
Abbots Habit was sent off 2-5 favourite at Ayr, but drifted to 1,000-1 in running during the race when he looked certain to be beaten.
One lucky punter had £3 on the horse, and he got up in the final stride to beat Dantor by a short head.
Big-priced winners
Arctic Blue, at 200-1, won at Chepstow in 2005. The Great Delaney (150-1) was another big-priced victor at the same track.
Beechy Bank, another 200-1 chance, won at Warwick in 2002.
A horse called Equinoctial was a 250-1 winner at Kelso in 1990.