Trainer Aidan O'Brien has admitted to some doubts regarding the stamina of the Air Force Blue ahead of the 2000 Guineas at Newmarket.

By far and away the clear form pick, he only met with defeat once in five races as a juvenile when second in the Coventry Stakes behind the Buratino who will be a rival on Saturday.

However, he turned that form around in the Phoenix Stakes and stamped himself as the leading two-year-old with a dominant display in the Dewhurst Stakes.

That was over seven furlongs (1400m) and he has another furlong to see out on his return to action.

"Obviously this will be the acid test as it's his first run over a mile, we'll probably learn a lot about him," O'Brien said.

"Those seven-furlong two-year-old races are like enhanced sprints and it is a tough searching mile at Newmarket, but you'd have to be hopeful that he will get it, however, you can never be sure until they do it on the day."