Floyd Landis has lost his appeal against a ban imposed after he failed a drugs test at the 2006 Tour de France.
Landis failed a drug test after the Tour's 17th stage
The American Arbitration Association (AAA) upheld the two-year suspension which will run until January 2009. Landis also forfeits his Tour title.
He was fighting a US Anti-Doping Agency (Usada) punishment after testing positive for synthetic testosterone.
The American has denied using drugs and alleged incompetence at the French laboratory which tested his sample.
Landis, 31, testified before a three-man panel in May, with his legal team claiming Usada's case against him was a "disaster".
He tested positive after his sensational win on the 17th stage of the 2006 Tour.
He got a highly qualified legal team who tried to baffle everybody with science and public relations, and in the end the facts stood up
Pat McQuaid, UCI president
Landis has one more avenue of appeal open to him - the Court of Arbitration for Sport based in Switzerland - and has a month to decide whether to do that.
"This ruling is a blow to athletes and cyclists everywhere," Landis said in a statement.
"For the Panel to find in favor of Usada when, with respect to so many issues, Usada did not manage to prove even the most basic parts of their case shows that this system is fundamentally flawed.
"I am innocent, and we proved I am innocent."
But Usada chief executive Travis Tygart said: "Today's ruling is a victory for all clean athletes and everyone who values fair and honest competition."
If Landis does not appeal, he will be the first person in the 105-year history of cycling's most famous event to lose the title because of a doping offence.
After the AAA's decision was announced, cycling's world governing body the UCI confirmed Spain's Oscar Pereiro, who finished runner-up to Landis, as the 2006 Tour winner.
"You never want to win a competition like that," said Pereiro. "But after a year and a half of all of this I'm just glad it's over."
Pat McQuaid, UCI president, said: "He (Landis) has been found guilty. It proves that the system works no matter who you are.
"We now await and see if he does appeal to CAS. It's not a great surprise considering how events have evolved.
"He got a highly qualified legal team who tried to baffle everybody with science and public relations, and in the end the facts stood up."