Trainer Jonjo O'Neill has withdrawn Merry King from next month's Grand National at Aintree.

The eight-year-old, who is a stablemate of favourite Shutthefrontdoor, has developed a breathing problem that will sideline the chestnut gelding for the remainder of the season.

"Unfortunately Merry King is going to miss the Grand National and the remainder of the season as a breathing abnormality has surfaced," the Racing Post quotes O'Neill as saying.

"After a routine piece of work we were not happy with the horse and the vets have discovered he may have developed a breathing problem.

"It's bad news for his owner Frank Gillespie and for the yard as he had been pleasing in his build up but now the problem has surfaced we have no choice but to miss the race."

Richie McLernon had been due to ride Merry King, but he is now expected to saddle a different contender.