Winner of the Queen Mary 12 months ago, Campanelle was in the firing line throughout under Frankie Dettori and while all her rivals were seemingly feeling the pressure in behind, Wesley Ward's filly was still travelling well.
However, suddenly Oisin Murphy's mount Dragon Symbol (4/1) quickened smartly and headed Campanelle, with the two then pulling clear of the pack.
With Campanelle (5/1) drawing back alongside Dragon Symbol, Archie Watson's three-year-old began to drift to his right, carrying the American across the track with him.
Murphy switched his whip into his right hand, getting his mount straightened up, and the two went at it hammer and tongs for the final half a furlong.
There was little between them as they flashed by the line together, with Dragon Symbol a head in front before the inevitable inquiry was called.
The stewards took their time before announcing the revised placing, with Ward in no doubt his filly deserved the victory.
He said: "I said earlier if we were in the States or Kentucky, we'd be drinking champagne already. Both horses ran fantastic, you can take nothing away from them, they dug down deep and were only inches apart there.
"I think it was a validated result as far as racing in the States is concerned, they came all the way across the track. Archie Watson came over and shook my hand - he's a true gentleman.
"She really is (good), especially being her first start of the year. For her to come since the Breeders' Cup, on heavy ground and to get through it and to have a performance like that is a true testament to how good she is.
"I actually thought (the rain) boded well for her chances. We didn't know if she would like this heavy ground but we knew that at least she did like the soft.
"Winning at Ascot, no matter how you get there, is special. These memories, especially with my children here, since the onset back in 2009 and to have my first winner, Strike The Tiger, take her to post is a very nice touch.
"(Ascot) has changed my life."
Dettori - winning the 76th race of his career at the Royal meeting - said: "It is never nice winning a race in the stewards' room. I feel sorry for Dragon Symbol's connections that I took it off them. But in these conditions, when you are taken across about eight lanes, I got nudged three times, lost my momentum and got beaten a head, so the stewards felt they gave me the benefit of the doubt.
"I was coming back on the line, and it was a good effort by Campanelle and by the original winner, but I'm glad that we've got it. I feel very sad that I got it, but I had reason to believe that my filly was still the best on the day.
"Campanelle won the Queen Mary, she won the Prix Morny, and now she has won the Commonwealth. They keep on bringing horses here and I'm always delighted to ride winners for Wesley.
"It leaves a bitter taste in the mouth because I didn't win outright, but that's the way things go - it's one lucky one that I got."
Murphy gained swift compensation by winning the following Coronation Stakes on Alcohol Free, but admitted his disappointment at the stewards' decision.
He said: "I'm very disappointed for the connections of Dragon Symbol, Archie Watson and his team, and Mr Kubota. I think there was a massive Japanese interest and unfortunately the horse finished the race in front, but it wasn't meant to be.
"It doesn't really matter what I feel, the stewards felt Campanelle deserved the race.
"I'm thrilled for Wesley Ward, he brought over a huge team. In fact, in his own words, Dragon Symbol was the best horse in the race, so does anyone else need an opinion? I don't think so."