Players were keen to back the jockey, who has a fantastic record at Royal Ascot, but no one was prepared for the amount of success the 48-year-old was going to have.
After Dettori produced four winners, his last two rides were significantly backed to go and make history. His fifth ride of the day in the Britannia saw Turgenev backed from 12-1 into 7-2 favourite, only to be beaten in the final stride.
His last ride, Questionare, was unable to land Dettori a fifth winner on the day, much to the delight of the bookmakers.
William Hill tweeted a second Gold Cup win for Stradivarius, Dettori's fourth winner, cost the operator more than £2.5m alone. The operator also tweeted a bet slip from one customer who scooped £12,000 from a £10 stake on Dettori’s "awesome foursome."
The operator's Communications Director, Ciaran O'Brien, told Gambling Insider: "When Turgenev was hugging the rail and staying on we were preparing for something the like of which we have never seen before. Then Harry Bentley stayed on for his first ever Ascot winner. Bentley will be the toast of bookmakers for many years to come."
With two more days of Royal Ascot to go, bookmakers will be hoping to avoid seeing Frankie Dettori in the winners' enclosure any more.
Racing Post's Head of Betting, Brian Morris, said to Gambling Insider: "Frankie's success was a joy to watch and was nearly one of the most historic days in racing history. With him being such a popular jockey it's understandable that punters follow him, especially at Ascot, so there would have been a lot of happy punters out there after the fourth race.
"If he went through the card I think it's certain it would have been a record losing day for bookmakers. However, a couple of big price winners at the end eased the pain of earlier in the day.''
Sky Bet's Head of Sports PR, Michael Shinners, said: "After the first four races we were looking down the barrel of the worst day in Sky Bet history. The final furlong of the Britannia when Biometric reeled in Turgenev represented the biggest swing in recent memory for the bookmaking industry.
"Despite the final two results going our way it was still very much a day for the punters and many of them go into today with plenty of money in their accounts."
