The heavily backed gelding Royal Wedding delighted punters by winning the Crabbie's Chase at Fontwell Park.
At the West Sussex track's Royal Wedding Race Night on Friday, the nine-year-old, who won at the course in February, came in at 4-1.
It left bookmakers predicting a multi-million pound payout on the victory.
The son of King's Best and grandson of Blushing Groom, Royal Wedding is owned by Sussex County Cricket Club chairman Don Trangmar.
The horse's price had been 8-1 on Thursday but it shortened after heavy betting from numerous occasional punters who had been intrigued by the timing of the race.
"Madame Fate has given us one hell of a kick in the crown jewels. Royal Wedding backers are over the moon," said David Williams from Ladbrokes.
Coral's David Stevens agreed and said: "Thousands of people placed bets on Royal Wedding, and the bookies will be paying out millions as a result.
"On the day Prince William and Kate Middleton tied the knot, there was only going to be one winner of the race, and Royal Wedding didn't let the side down, becoming the most aptly-named winner since Party Politics won the Grand National just five days before the 1992 General Election," he said.
The Nick Gifford-trained Royal Wedding was in front from some way out but began to toil over the last three fences, making a notable slip at the second-last.
However, he dug deep after the last and, with jockey Liam Treadwell, negotiated the climb to the line to repel Take A Mile by 12 lengths.
"I'm delighted and a bit relieved he won after we've had so much publicity," said Gifford, who trains at nearby Findon.
"We just didn't want him to do anything stupid like fall at the first or anything like that.
"He's not been the most consistent horse, but we are absolutely thrilled with the result."