Tour de France chiefs say they will be angry and sad if 2006 winner Floyd Landis' failed drugs test is confirmed.
Landis could be stripped of his title if his positive test is confirmed
Landis' Phonak team revealed that the American had tested positive for the male sex hormone testosterone.
As well as a two-year ban, Landis faces being stripped of his victory if his B sample matches the first test.
"If the result is confirmed, anger and sadness would be the dominant feelings for those thrilled by the 2006 race," said the organisers in a statement.
The 30-year-old Landis tested positive after his sensational win in the 17th stage, which he won by powering away on a 130km solo breakaway.
If the counter-analysis confirms the positive he must be punished
Phonak director Juan Fernandez
The announcement is the latest debilitating blow to cycling, which is seeking to clean up its tarnished image.
It follows the withdrawal of pre-race favourites Jan Ullrich of Germany and Italian Ivan Basso before the start of the Tour after they were linked to a Spanish doping investigation.
"Now more than ever the organisers will maintain the firm attitude they have displayed since the start from Strasbourg," added the statement.
"No matter how harrowing this news is for cycling, it nevertheless illustrates that the fight against doping by the Tour de France together with the teams and the sponsors is gaining ground in an irreversible way."
If Landis is stripped of his title, 2006 Tour runner-up Oscar Pereiro is expected to be declared the winner.
Pereiro (R) is not celebrating Landis' positive test
"Until the UCI confirm it, it isn't possible to say anything, despite what Phonak have said," he said.
"In any case, I have a bittersweet feeling because it is bad news for cycling, and I would prefer to remain second and that they don't confirm the positive.
"Should I win the Tour now it would feel like an academic victory. The way to celebrate a win is in Paris, otherwise it's just a bureaucratic win."
French rider Cyril Dessel, who finished seventh in this year's Tour, revealed his surprise at the announcement.
"I am shocked by this revelation. I don't know what to say," he said.
"We should not draw any hasty conclusions, we must wait for the result of the sample B of course. But if it turns out to be true, this is going to be another huge blow to cycling."
We are going to think about our future as broadcaster
German television station ZDF
Triple Tour de France winner Greg LeMond expressed anger upon hearing the news of Landis' test.
"I'm devastated and extremely disappointed," he said. "I can't imagine the disappointment for Floyd and his family.
"I still believe it was one of the cleanest Tours ever. But is it 100% clean? No.
"You will always find riders who transgress the laws. I really did believe Floyd was not among them, that he was clean."
Phonak have suspended Landis pending the results of tests on the B sample and the team say they will sack the 30-year-old if the positive test is confirmed.
"We will have to wait for the counter-analysis and see if this gives a positive as well," Phonak director Juan Fernandez told Spanish radio.
"It is very bad news. It is very sad. These types of things are really bad but if they happen we have to face up to them.
Fernandez will take a hard line with Landis if the test is confirmed
"If a rider does something wrong or bad he has to be dealt with.
"If the counter-analysis (B sample) confirms the positive he must be punished. He will need to be expelled as has happened in other cases."
US Olympic Committee chairman Peter Ueberroth says more needs to be done in the battle against doping.
"The bottom line is this, while good progress has been made in the fight against doping in sport, we believe that much more needs to be done," he said.
"If the B sample confirms the result of the A sample, and if Floyd Landis is ultimately found to be guilty of a doping offence, it would be an incredible disappointment."
USA Cycling spokesman Andy Lee refused to condemn Landis, saying that he would wait for the B sample results.
"Because it's an anti-doping matter we can't really comment on it," he said.
"Out of respect for both the process put in place and the rights of Floyd as an athlete we need to let the process run its course before we can make any type of comment."
Meanwhile German television station ZDF said it might now cease broadcasting the Tour de France.
"We signed a broadcasting contract for a sporting event, not a show demonstrating the performances of the pharmaceutical industry," said editor-in-chief Nikolaus Brender.
"We are going to think about our future as broadcaster and maybe refuse to broadcast this event."