The 53-year-old, who rode Red Rum to his historic third Grand National victory in 1977, is suffering from a viral infection and was taken to hospital in County Cork at the weekend.

A spokeswoman for the Thomastown Castle yard, where Stack works as a trainer, said on Monday: "They have told us he is stable this morning but that's all we know."

Stack was twice champion jump jockey in 1974-5 and againt two years later.