Eric Libaud's four-year-old was at the rear of the field a furlong out before jockey Olivier Peslier, in the saddle for the first time ahead of regular rider Ioritz Mendizabal, grabbed the lead inside the final furlong.
The 4-1 shot came under a strong challenge from the 6-4 favourite in the closing stages but held on by half a length with another French raider, Never On Sunday, a head away in third place.
Tazeez made the early running from Twice Over and Virtual with Tartan Bearer on the outside. Twice Over went on two furlongs out but he was soon strongly challenged.
Peslier's patience paid off on last year's French Derby victor - without his usual tongue tie which was not declared - who came through with a powerful run.
He got first run on the second and third to become the second French-trained winner in three years after Manduro in 2007. Twice Over rarely enjoys much luck in big races and having looked like winning at one stage he had to settle for fourth, beaten less than a length.
Libaud said: "Of all the winners I've had in the world this is the best one.
He got first run on the second and third to become the second French-trained winner in three years after Manduro in 2007. Twice Over rarely enjoys much luck in big races and having looked like winning at one stage he had to settle for fourth, beaten less than a length.
Libaud said: "Of all the winners I've had in the world this is the best one.
"I knew it was going to be difficult from last place but I could see the horse was going well and Olivier had told me the straight at Ascot is long, so I was quietly confident.
"This is the nicest and best victory I've ever achieved." Peslier added: "I was just behind the lead. My horse is not lazy and I just took a bit of time. I needed a chance and he moved out well.
"The trainer told me before it was better if he came late and the race was perfect for me.
"He's a typical French horse. When you are one or two lengths behind it's not a big problem because he's got a good turn of foot. That was a big help for me."