Irish horses racing in the Grand National could be quarantined for up to six months.

Irish Agriculture Minister Joe Walsh had originally banned horses from travelling to Britain to prevent the risk of spreading foot-and-mouth.

He announced on Monday this ban was to be lifted but insisted horses be quarantined for an unspecified period.

"The horses must stay in the UK for an as yet unspecified period after the last confirmed case of foot-and-mouth," Aintree spokesman Nigel Payne said.

Rough sea conditions prevented the six Irish horses, including last year's winner Papillon, being shipped over to the UK and will now fly from Dublin to Liverpool on Friday.

"They were due to come over (on Thursday) but we would bring them over in a tardis if necessary," Payne said.

He also confirmed the three-day Grand National Festival had benefited from the cancellation of the Cheltenham Gold Cup due to the foot-and-mouth crisis.

"The Cheltenham effect has massively boosted advance bookings, although we are still down by about 5%," he said.

"People have said 'Aintree is on so we'll go to Aintree'."

He added that visitors had happily accepted security precautions against foot-and-mouth, such as walking over disinfectant-soaked mats.

"If you come to a major race meeting wearing expensive Gucci shoes you deserve to get them dirty," Payne said.

"The two most important things are to check the spread of foot-and-mouth and to ensure the safety and comfort of our visitors."