PARIS — Former world cycling champion Tom Boonen was barred today from this year's Tour de France after testing positive for cocaine.

Tour organizers said the rest of Boonen's Quick Step team remains eligible to compete in the race.

The Belgian rider apologized to fans earlier today in Brussels, but did not confirm or deny allegations he had taken cocaine. Belgian prosecutors opened a probe Tuesday into possible cocaine possession by Boonen.

Boonen, world champion in 2005, earned the green jersey as the best sprinter in last year's Tour.

Today's decision means two of the top contenders are now out of the race, which has been riddled with drug problems in recent years. Defending champion Alberto Contador will not have a chance to defend his title because his Astana team has been banned from the race due to a team doping scandal before his arrival.

Race director Christian Prudhomme said by telephone that Boonen was barred from the race and that the drug accusations against the former world champion are "an attack on the integrity of the image of the Tour de France."

"The emblematic world champion that he is should set an example," Prudhomme said. "I told him that for us, he could not be there for the start of the race in three weeks if what has been announced turns out to be true."

Still, he stressed, "We are not in a case of doping, he did not take cocaine to be ahead of the others."

One of cycling's most gifted sprinters, Boonen won the prestigious Paris-Roubaix cycling classic on April 13, beating former champion Fabian Cancellara of Switzerland in a sprint finish. Boonen was world champion in 2005 and won several sprint stages on that year's Tour.

"I am not going to defend myself here today, but I have caused my family, friends and team pain and I apologize for that," Boonen said in his apology. "I have been in the news in a negative way lately. I am not perfect and will accept the consequences."

The cyclist has garnered celebrity status in Belgium, where cycling is one of the most watched and followed sports. He urged his fans "to keep faith" and said he would be back soon after a break from racing.

Quick Step lamented the decision to keep Boonen out of the Tour.

"The team is sorry to have not been able to meet the representatives of (the Tour organizer) before the decision was taken, considering that the result of the test undergone by Boonen won't have any consequence on a professional and sporting level," Quick Step said in a statement.

The World Anti-Doping Agency classifies cocaine as a stimulant, but it is only prohibited if the substance is taken during a competition period.