The Ballydoyle trainer blames the most recent defeats of his two Classic-winning colts on health issues which afflicted his string at the end of the summer

Aidan O’Brien is forecasting an upturn in form from Constitution River and Gstaad when they line up for their races at Qatar Racing British Champions Day on Saturday.

The duo are among the cast of stars seeking success at Ascot when it is the venue for the richest single raceday in Britain, and grand finale to the Flat racing season. Constitution River contests the £1.3 million Champion Stakes and Gstaad the £1.1m Queen Elizabeth II Stakes.

During the first half of the season Constitution River won the French 2,000 Guineas, French Derby and Coral-Eclipse Stakes in the manner of another Ballydoyle superstar.

Gstaad won the Irish 2,000 Guineas, between three narrow defeats at the hands of the outstanding Bow Echo.

However in their latest efforts, Constitution River has suffered back-to-back defeats in the Juddmonte International and Irish Champion Stakes and Gstaad disappointed in the Prix du Moulin.

O’Brien subsequently blamed their below par performances on a low-grade infection in the yard at the time but he is confident both are over their ailments.

“At that time of their last runs, there were four or five Group 1s that the horses weren’t finishing and they were finishing second,” he said.

“All of a sudden there was an outbreak of coughing in the yard and usually when they start coughing it’s the end of everything they had. There were SAAs [Serum amyloid A] in their bloods which were spiking up for two or three months and disappearing for no reason.

“That usually means infection but we couldn’t find any infection and they were coming and going. So it was probably hanging around there for a long time. It’s their last runs where they were most affected.

“We gave a lot of horses a course of antibiotics since and we think that’s helped them and we are very happy that their form has come back to where we think they should be.”

O’Brien will have eight runners at the meeting versus five from Andrew Balding, who holds a near £100,000 lead over his rival for the British trainers’ championship.

Balding’s main hope will be Kalpana who bids for a third victory in the Fillies & Mares on what is likely to be the final start of her career.

Although the outcome of the trainers’ contest is not determined until December 31, a Group 1 success for either could be decisive with O’Brien ruling out having any runners after the end of the Flat Turf season.

“It is a privilege and pleasure for us to be involved in the championship,” he said. “It hasn’t affected our running plans. We always put the horses first. If it happens we would obviously be over the moon.

“We’ll do our best but after this we’ll only have the Futurity left and then we’re finished. For us to do it we would have to win one of the Group 1 races and that’s not easy.”