Last year's winner was found to have heat in a leg following routine exercise today and is out for the season.

"Unfortunately after exercise this morning he was found to have heat in his leg and will be out for the season, " said trainer Mouse Morris.

"Everything has hit me, as I lost my mother last night as well. I've got to get my head around it all myself. I'm devastated.

"The horse has heat in his leg, but was sound. He is not lame or anything. We had it scanned and he needs to rest.

"Hopefully, he'll be back next season."

The eight-year-old is owned by Ryanair boss Michael O'Leary under the banner of his Gigginstown House Stud.

In a statement, O'Leary said: "I would like to thank Mouse Morris and his team at Everards Grange for the great work they have done with War Of Attrition during the season and I am desperately sorry that despite all their hard work he won't be returning to defend the Gold Cup this year.

"We all hope that with sufficient rest and treatment he will return to racing next year. We know he couldn't be in better hands than with Mouse and his team.

"Both my wife Anita and I would like to send out sincere condolences to Mouse Morris, whose mother passed away late last night."

War Of Attrition, who had been second-favourite at around 5-1 to repeat his 2006 triumph, was third to The Listener in the Lexus Chase at Leopardstown in December on his latest outing.

Following the shock news, market leader Kauto Star was cut to 5-4 from 6-4 with the sponsors, who go 4-1 from 5-1 Exotic Dancer and 7-1 from 8s The Listener.

Ladbrokes make Kauto Star an 11-8 chance, with Exotic Dancer 4-1 and The Listener 7-1.