World marathon record holder Paula Radcliffe said on Thursday she had a stress fracture of her left femur and that some specialists had told her it would be impossible for her to compete in the Beijing Olympics.
The 34-year-old, who made the announcement via a teleconference, said she was still "90 percent" confident she could be fit but would only go if she felt competitive.
"It's not good news and not the news I wanted to hear, but I will give myself every chance," said Radcliffe, who has been on crutches with what was originally thought to be a hip injury.
Radcliffe said it was only when she eventually had a bone scan after two MRI scans on her hip that it was discovered she had an "atypical femural compression" rather than a hip injury.
"The bottom line is it's not ideal and some specialists have said with the training I need to put in it would be impossible," she said. "I will give myself every chance but I won't run through pain and I won't go if I feel I can't be competitive.
"It's going to go down to the wire and I will need a bit of luck. It has been a nightmare last three weeks, extremely emotional and extremely hard.
"It's bad enough when it's an Olympic year but it's even worse when you don't know exactly what you're dealing with."
Radcliffe said she was training in the swimming pool and could be back running in two weeks but would only have a clear indication of her Beijing chances once she raised her workload.
"The crucial period would be building up to the high mileage," said Radcliffe, who missed this year's London Marathon with a toe injury and who failed to finish the 2004 Athens Olympic marathon.
"I will have to be totally honest with myself because you lose more than you gain by trying to run when you shouldn't."
Radcliffe described some of the tests on her hip as "scary" and revealed that one specialist even suggested a risk of bone cancer, although that was quickly ruled out.
If she is forced to withdraw, the 2005 world marathon champion, who gave birth to a daughter in January last year, said she would not rule out running at the London 2012 Olympics.
"I still think that is a viable option for me," she said. "Marathon runners can still be in good shape at 38."
