Westover made the trip to France worthwhile by winning the Grand Prix de Saint-Cloud for Ralph Beckett and Rob Hornby.
Winner of the Irish Derby last season, he had run two fine races in defeat this year behind Equinox in Dubai and Emily Upjohn at Epsom.
Sent off favourite to add a second Group One to his tally, he raced in third as Tunnes and Malabu Drive, Westover’s pacemaker, ensured a stiff gallop.
Hornby was keen to put the Juddmonte-owned colt’s stamina to good use and took over at the head of affairs with fully two furlongs to run, but was made to work hard as Christophe Soumillon appeared to be travelling the better on Zagrey, who was one place behind Westover in Dubai and having his first run since.
Westover, to his credit, galloped strongly to the line and ultimately ran out a clear-cut winner by two lengths.
Betfair cut the winner to 6-1 from 10s for the King George VI and Queen Elizabeth Stakes at Ascot later this month.
Hornby told Sky Sports Racing: “Winning a Group One in these colours is the stuff of dreams.
“I was brought up watching these colours win prestigious races and to be able to do it at Group One level is the highest honour and I’m grateful to everyone who has helped me along the way.
“Ralph has trained this horse so well from two to three and three to four, so to win a Group One at four is so important. These are things you dream of, so when they come true you don’t know how to think.”
Of the race, he added: “The pace was really good early on, Dougie (Costello on Malabu Drive) was able to keep it honest the whole way.
“When he dropped the bit he was able to work into it but funnily enough, once he shut down he took some stoking up again, so I was keen to get on my bike early. He hit the line strong and the time matches that.
“This is my third Group One for Ralph and fourth overall but it’s nearly a year since my last one and they are so important for a jockey.”
Beckett said: “Inevitably there was pressure as it was a long time since he won, over a year, and the longer that goes on the harder it gets.
“There was nothing wrong with his two runs this year, he’s only had the one blip in the King George. He was great today and it was a very fast time.
“For so long we had a reputation for training long distance fillies, so it’s important to change the dial, as Angel Bleu has done as well.
“I think we’ll either stay at a mile and a half or even have a conversation about going up in trip, because the peloton came to him but he stretched away in the last half a furlong.
“We’ll enjoy today and see what comes next, this was important.
“We’ll certainly consider the King George and see how he is closer to the time. There will be a bit of a monkey on the shoulder saying is this a good idea after what happened last year but we’ll see.
“The Arc will be under consideration too, he ran a great race in it last year (sixth) but got stuck in the ground.”