Aristotle hit hard by left-leaning tendency.
Last year's Racing Post Trophy winner came home two lengths clear of Rhenium in the Group Two event over ten-and-a-half furlongs but was demoted by the stewards for interfering with Boutron early in the straight.
Michael Kinane's mount suddenly jinking violently left when challenging Sobieski and interfered with Boutron, ridden by Tony Junk.
Despite Kinane's attempts to keep the previously unbeaten colt straight on the heavy ground, Aristotle continued to veer sharply left in the closing stages.
It was estimated that he lost about six lengths in the manoeuvre and it is to his great credit that he still had two lengths to spare over second-place Rhenium at the line.
After a relatively swift inquiry Aristotle was demoted to third with the Jean Claude-Rouget-trained Rhenium (Thierry Jarnet), being awarded the near-pounds 30,000 prize and Boutron also placed in front of Aristotle.
Kinane said: 'I tried to correct him but there was nothing I could do. It is extraordinary because he has never done that before. At least I must be happy because he got the trip well in the bad ground.'
The stewards took no action against Kinane, deeming that he made every effort to correct the horse.
The setback continued the gloomy start for O'Brien's three-year-old stars, especially as Royal Kingdom had earlier struggled in the rain-drenched ground to finish a disappointing fifth, beaten about two-and-a-half lengths by Berine's Son in the Group Three Prix de Fontainebleu.
Last year's Prix Marcel Boussac winner Lady of Chad stretched her unbeaten tally to three when gaining a cosy neck victory over Texalani in the Prix de la Grotte.
Barry Hills will work three of his leading stayers after racing at Kempton today.
Rainbow High (to be ridden by the trainer's son Michael) will gallop in preparation for a bid to repeat last year's win in the Tote Chester Cup.
He will be joined by Yorkshire Cup-hope Rainbow Ways (John Reid) and Elmutabaki (Richard Hills), who has not raced since finishing eighth in the St Leger at Doncaster last September.