The key question before the Prix de l’Arc de Triomphe here on Sunday was whether the explosive finishing speed of the favourite, Ace Impact, would be as potent in the unknown territory of a mile-and-a-half, two furlongs further than any of his previous runs. The answer was emphatic and incontestable, as Cristian Demuro brought Ace Impact with an electrifying run from the back of the field, chasing down Westover inside the final furlong before crossing the line nearly two lengths up, and still unbeaten after half a dozen starts.

Only two of the 15 runners – Onesto and Through Seven Seas, who went on to finish third and fourth – were behind Ace Impact as the field turned for home with just over two furlongs to run and Demuro shook the reins and asked him to quicken.

The response was so abrupt that by the time they reached the furlong pole just over 10 seconds later, the eventual result was in no doubt. Westover, the King George runner‑up, was still about a head in front, but Ace Impact was finishing to such effect that he was ahead a few strides later, and Demuro was up in his irons and waving his whip in triumph as they passed the post.

Ace Impact returned to the unsaddling enclosure as the first unbeaten Arc winner since Treve in 2013 and the first French Derby winner to follow up in the Arc in the same season since the trip in the Classic at Chantilly was cut from 12 furlongs to 10 in 2005.

While it has not been possible to trawl through 103 years of Arc history to be certain, Ace Impact is probably the first to have started their Arc‑winning season in a maiden at Cagnes-sur-Mer in mid-January. If this turns out to have been his one and only year on the track, as seems quite possible, it has at least been as unusual as it has been unforgettable.

This was a second Arc success in four runnings for both Demuro and Jean-Claude Rouget, the winner’s trainer, who teamed up to win in 2020 with Sottsass.

“I am like a very backward horse,” Rouget said in the winner’s enclosure. “I am just maturing now, so I hope I have a few good years again in front of me. I started with jumpers and very bad Flat horses. With Millkom [a Grand Prix de Paris winner] it was the first sign of a Classic career in 1994 and now we have a very strong stable.”

Of the winning horse he said: “I think it is his acceleration [that makes the difference], it is something I’ve never seen. I don’t know if he will go straight to stud or run next year, it will be decided between the two owners and me.”

Demuro said afterwards that he had been confident throughout, despite sitting close to the rear of the field. “I knew when we came into the straight that I could get close to the pace, and that when I pushed the button, he was going to accelerate, and that’s what he did.”

Demuro performed a pared-back version of Frankie Dettori’s famous flying dismount as he returned to the winner’s enclosure – a tribute, he said, to a jockey who will now retire with a record six Arc victories to his name from 34 starts in the race.

“I had no pressure today,” Dettori, who finished 13th of 15 on Free Wind, said. “The Arc has been a great race for me, a good journey, but I’m bowing out with a smile on my face. Six Arc wins, what can you say? I’ve ridden some great champions and riding in it 34 times is a record in itself.”

Both the Breeders’ Cup Turf (definitely) and the Japan Cup (probably) were ruled out as possible targets for Ace Impact this season, and the Champion Stakes at Ascot this month would probably come too soon after this huge effort. As a result the betting has to be that we have seen the last of him on the track, hugely welcome though a four‑year‑old campaign would be for a horse who has been racing for less than nine months.

Westover, though, could well be making the trip to Santa Anita next month, having run yet another excellent race on his ninth consecutive start in a Group One to take second place.

“What a run, I’m delighted,” Ralph Beckett, his trainer, said. “At the top of the straight I thought we were in business, but there was just one better.

“I’ve always fancied the Breeders’ Cup Turf for him because he’s a horse who enjoys his time away, as we saw in Dubai [earlier this year]. It is shaping up to be the best [Turf] ever, but that’s life and I think we will probably go.”