Murtagh steps in for Fallon, who is banned until August 2009

Murtagh will ride for top Irish trainer Aidan O'Brien, for whom he won the Epsom Derby on High Chaparral in 2002.

Speaking from Dubai, Murtagh, 37, said: "I am over the moon. It is a great opportunity and I am really looking forward to riding for the team."

Fallon was banned for 18 months after failing a drugs test in France.

Interview: Johnny Murtagh

The move, affecting one of Flat racing's most coveted jobs, places some doubt over Fallon's long-term future.

A statement from Coolmore said: "Owners John Magnier, Michael Tabor and Derrick Smith are delighted to announce that Johnny Murtagh will be the retained jockey for their horses at Ballydoyle and elsewhere for the 2008 season."

The Coolmore trio stayed loyal to Fallon when he faced a race-fixing trial at the Old Bailey.

He was acquitted of all charges, but news of his failed drugs test emerged the day after he walked free from court.

Murtagh, like Fallon, is a three-time Epsom Derby winner, with victories on Sinndar (2000), High Chaparral and Motivator (2005).

He told BBC Radio 5 Live: "It's the biggest job in racing, especially in Europe it's right up there. They've got a great team of horses, a great team of people and i'm looking forward to slotting in there."

"I do think there would be no problem for Fallon to come back, though."

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A triple former Irish champion, he has bounced back from weight problems to become one of Flat racing's most respected riders.

Magnier, Tabor and Smith are the men behind the Coolmore Stud, which employs O'Brien as trainer at Ballydoyle in County Tipperary.

O'Brien said: "Johnny has already enjoyed plenty of success as part of the team at Ballydoyle and we are delighted to have him on board."