William Hill estimated that the industry could have lost £5 million if Scottish singer Susan Boyle had taken the title on Saturday.
The lion’s share of bets were placed on the favourite from Blackburn, West Lothian. One supporter in Devon, backed her to the tune of £7,000.
Spokesman Rupert Adams said: “We were expecting to pay out millions of pounds, but after the biggest shock in reality TV betting history we have won a six-figure sum.”
Darren Haines from Paddypower said the final of any TV competition attracts new one-off voters who can produce a shock result.
This was seen when Will Young won Pop Idol in 2002, despite Gareth Gates being widely expected to take victory, he explained.
But Adams still believes that Boyle, 48, will have a strong career and is more likely to have a number one chart hit in America than in Britain.
Britain’s Got Talent fans placed more than £3m in bets on the outcome of the competition, with William Hill taking 12 bets per second at the peak on Saturday night.
In terms of Susan Boyle’s future now, the bookmaker gives odds of 3/1 that her first single will reach number one, but 5/2 that her success will be across the Atlantic. There are also strong odds that her first release will be the best-selling single of this year (3/1) and that she will perform in the West End (4/1).
Publicity surrounding Britain’s Got Talent in the last few days saw the industry take millions in wagers.
Reality television programmes have become big business for bookmakers, and the ITV show would be even more popular if the finalists were known at an earlier stage in the competition, Coral spokesman Simon Clare said.
The firm had one of the biggest days of the year on Saturday with the FA Cup final coinciding with BGT.
He said: “Five years ago the idea of a TV programme competing with the FA Cup final would have been laughable, but now we’ve given them equal space.”