Japan had been a general 3/1 shot to win Ascot's £1.4million Group 1 on Saturday, but when outlining his running plans for British Champions Day, his trainer Aidan O'Brien revealed he was a doubtful starter.

The reason behind the doubt is unclear, though O'Brien warned it could be enough to rule him out of the British Champion Stakes, the trainer having already said the Prix de l'Arc de Triomphe fourth will not go to Santa Anita for the Breeders' Cup in November.

"We'll be making a decision regarding plans for Japan in the next couple of days, but it's quite possible he won't run again this year," O'Brien told the Racing Post. "It looks as if Magical and I Can Fly might run in the Champion Stakes. Magna Grecia is going for the QEII, while Circus Maximus will probably be kept for the Breeders' Cup Mile."

Fleeting Gets Green-Light For Fillies & Mares

O'Brien's plans caused a shake-up in the betting for the Champion Stakes, with Magical now a very solid 9/4 favourite, and Japan out to as big as 5/1.

Magical is also entered in the Champions Fillies & Mares Stakes, a Group 1 she won last year, but now Fleeting, runner-up in the Prix de l'Opera at Paris-Longchamp, is set to be O'Brien's main fancy there, with Delphinia and Pink Dogwood likely to join.

Magical is 8/1 in that ante-post betting market, the Prix Vermeille, Irish Oaks and Ribblesdale heroine Star Catcher holding sway at the top of the betting with odds no bigger than 15/8. She is ahead of Fleeting, now into 5/1, and the John Gosden-trained Anapurna.

O'Brien also pointed Kew Gardens, Capri and South Pacific towards Ascot's Long Distance Cup, and added that So Perfect would contest the Champions Sprint, which has seen strong support for Addeybb. 1,000 Guineas winner Hermosa is unlikely to take up any of her entries on British Champions Day.

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Japan Decision Revealed On Thursday

Final fields for all races on Ascot's stellar Saturday card will be revealed after 10am on Thursday, and it will be understandable if Japan is not among them.

The three-year-old struggled with form early in the season, but bounced back to win at Royal Ascot, the Grand Prix de Paris at Longchamp, and the International Stakes at York, all on ground no worse than good to soft.

He was beaten four lengths by winner Waldgeist in a very soft Arc only two weeks ago, and has proven himself better over 1m4f than the British Champion Stakes' 1m2f. After six races in six months, it could well be time to take a break.