Kenya's 1500m Olympic silver medallist Bernard Lagat has become a US citizen.
Lagat will have to wait to compete for the US
Lagat, who won silver in Athens last August and bronze in Sydney in 2000, has lived in the United States since 1996 and has a home in Tucson, Arizona.
The decision means Lagat will not be able to compete at the World Athletics Championships in Helsinki in August.
He said: "I thought long and hard before changing my nationality but, at 30, I have to look to my future after my running career has come to an end."
Lagat could be free to represent the US within 12 months if Athletics Kenya agrees to his transfer, although they could decide to enforce a three-year transition period.
"I hope to continue running at the very highest level until at least the 2008 Olympics," said the Washington State University graduate, whose 1500m personal best of three minutes 26.34 seconds is the second fastest in history.
"I realise that changing my nationality makes me ineligible to compete at the 2005 world championships.
"But I look forward to trying to run as fast as possible in other meetings this summer, with the aim of inspiring and uplifting Kenyan and American athletics fans alike.
"The decision to change my nationality was not one I took lightly as I am very proud of my Kenyan heritage."
Lagat is the latest high-profile Kenyan athlete to switch allegiance.
Athletics Kenya chairman Isaiah Kiplagat said: "We're shocked at Lagat's decision to change his nationality at a time when we're preparing for the World Athletics Championships.
"Although Kenya has many talented runners, Lagat was currently the best in the 1500m.
"He can't run for the US now in any major championship like the world championships until after the expiry of three years according to the new rules.
"That can be reduced to one year if the US federation and ours agree. But he's free to take part in the Grand Prix meetings as an individual."
The trend of Kenyan athletes changing nationality has prompted sports minister Ochillo Ayacko to call for all defectors to be banned from training in Kenya.
But Kiplagat said: "It's not possible since, like any other foreigner, they have a right to move anywhere.
"That's our position as the body in charge of athletics."