Triple-jumper Yamile Aldama has agreed to compete for Sudan after halting her bid to obtain a British passport.
Aldama had been tipped to win medals for Britain
The Cuban-born athlete, favourite for Olympic gold at Athens, married a Scot and spent two years trying to get the Home Office to grant her citizenship.
But she will now go head-to-head with Britain's Ashia Hansen in Greece.
"It's a real shame what has happened, but according to the Home Office there are no exceptions to the rule," said UK Athletics chief Dave Moorcroft.
"I can understand why Yamile's done this and wish her well in the future."
Aldama has guaranteed an appearance at the Olympic Games by becoming a national of Sudan, despite living London.
She had been expected to eventually get a British passport, but had been told that her request would not be granted ahead of the usual three-year waiting period.
"Effectively it is a free transfer for her," said a close friend of Aldama.
"She had become totally disillusioned that no-one wanted her to compete in a Great Britain vest. In the end she would have jumped for anyone."
Aldama will compete for her new country at an international indoor meeting in Belgium on 8 February.