British cyclist David Millar has fractured his right collarbone in a crash while competing in Belgium.
The 35-year-old was taking part in the E3 Harelbeke race for his team Garmin-Barracuda.
Team doctor Kevin Sprouse said: "David has fractured his right clavicle. This is the same one he's broken before."
A team spokesman said it was not known how long he would be sidelined for.
Sprouse added: "His health is our top priority and we'll continue to evaluate him to determine the best next steps. David is doing well and in good spirits."
Millar is currently banned from competing at this summer's 2012 Olympics because of a British Olympic Association (BOA) by-law which prevents drug cheats from competing at the Games.
However, the by-law has been challenged by the World Anti-Doping Agency (Wada), who say it is not compliant with the Wada code and is therefore unenforceable.
A special hearing by the Court of Arbitration for Sport (Cas) began earlier this month, with a decision on the matter due in April.
Friday's 203km E3 Harelbeke, a one-day World Tour race, was won by Belgian Tom Boonen. The Omega Pharma-Quickstep rider edged out three-time world champion Oscar Freire in a sprint finish.