UK Athletics head coach Charles van Commenee says Paula Radcliffe's chances of challenging for an Olympic medal in London are only 50/50.
Radcliffe, the world record holder in the marathon, has been struggling with a foot problem.
"Paula is always a concern," he said. "She is on a thin line every day for the last decade. Nothing has changed."
Asked if Radcliffe would be fit enough to compete in London, he added: "Shall we flip a coin? Paula is Paula."
The 38-year-old won marathon gold at the World Championships in 2005, but has never won an Olympic medal.
She failed to finish both the marathon and 10,000m in Athens in 2004 after being hampered by illness and injury. She did finish the marathon in Beijing in 2008, but another leg injury saw her finish down in 23rd.
Speaking earlier this month, Radcliffe revealed that the foot problem which forced her to miss the London Marathon in 2008 had flared up again.
"Preparations for the Games have gone really well and I was really pleased until the last 10 days or so," she said at the time. "The joint in my foot that can give me trouble has been giving me a bit of a problem.
"I don't think my participation at the Games is under threat at this stage. It's just a case of managing it. I can run on it, it just hurts."
Radcliffe achieved the Olympic qualifying time in Berlin in September last year, but in her only race in 2012 ran her slowest-ever half-marathon time in Vienna in April.