The Group 1 National Stakes winner was due to be making his seasonal reappearance in the Irish Classic, but has been ruled out by a 'little setback'. 

The Wootton Bassett colt has been impressive in his two winning runs over seven furlongs at the Curragh and was the ante-post favourite for the Irish 2000 Guineas. 

O'Brien said, "He has just had a little setback training. Hopefully, it is nothing too serious and he will be back later in the summer.

"He has options at Ascot and further afield than that as well.

"These things happen, so it's a long way from a problem. It is obviously disappointing for the owners. This race has been his target for the last while, but we will look forward to things later in the summer.

"Timing-wise Royal Ascot will be fine, but we will have to decide whether we want to run there or in other races. We'll not be rushing into any decision."

Al Riffa holds entries in the Group 1 St James' Palace Stakes at Royal Ascot, the Group 1 Irish Derby at the Curragh and the Group 1 Coral Eclipse at Sandown Park.