Sprinter Sacre romped to victory in the Queen Mother Champion Chase as thousands of fans flocked to the west country for the highlight of the National Hunt racing calendar.
The bookmaker had offered Evens last Thursday about the unbeaten horse as part of their biggest ever promotional campaign.
Thousands snapped up the odds and were able to celebrate after the horse sauntered to victory by 19 lengths at odds of 1/4.
He was the shortest-priced Festival winner since Arkle won his third Gold Cup at odds of 1/10 in 1966.
"The Cheltenham Festival is the biggest week of the year as far as betting is concerned, and our offer of even-money about the hottest favourite at the meeting since the mighty Arkle in the 1960s proved too good to ignore," said Coral's David Stevens.
"The offer proved a huge hit, with many people comparing it to finding money in the street.
"When we began the offer last Thursday, we said we were willing to lay it lose a million, which we duly did.
"To be honest we never looked like keeping the money, as the horse ran out one of the easiest winners ever seen at the Festival."
Winning jockey Barry Geraghty described Sprinter Sacre as the "Frankel of jump racing" after a scintillating performance by the seven-year-old who destroyed Sizing Europe, twice a Festival winner, in the two-mile contest.
His stamp of authority drew comparisons with Frankel, who retired unbeaten last year after winning 14 flat races and is the highest-rated horse in the modern era.
"He's a class act. I think the 'Frankel of jump racing' is a fair way to describe him," Irishman Geraghty told reporters.
Sprinter Sacre is now rated as the third best chaser in history.
"It was like Barry Geraghty pressed the accelerator on a sports car. It was one of the most wonderful things I have seen in sport," said winning trainer Nicky Henderson.
"It does make you speechless. Sprinter Sacre is scary. He has that aura about him and is totally and utterly unique.
"He finds it ridiculously easy - he just loves doing it."
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