Tyler Hamilton, the captain of Switzerland's Phonak cycling team, faces a possible two-year ban after a second doping test from the Tour of Spain came back positive.
But the Olympic time-trial champion has been told he can keep his gold medal after a laboratory blunder ruined the results of a drug test at the Athens Games.
Earlier this week it emerged that Hamilton had failed two separate doping tests at the Athens Olympics and at this month's Tour of Spain.
But on Thursday the International Olympic Committee (IOC) said a back-up drug test had proved inconclusive and Hamilton could keep his medal.
Hamilton tested positive for signs of blood doping in the initial “A” sample on August 19 after his victory in the time-trial race in Athens.
But the IOC said analysis of the “B” sample had failed to confirm the original finding because of a “lack of enough intact red blood cells”.
IOC medical commission chairman Arne Ljungqvist said the Athens lab had made a mistake by deep-freezing Hamilton's blood sample rather than simply refrigerating it.
As a result, the specimen deteriorated and the B sample could not be analysed, he said.
“It was a case of human error, an unfortunate accident,” said Ljungvist.
Asked if Hamilton had been lucky to slip through the net, he said: “It's up to everyone to draw his own conclusions on that.”
Although in the clear on one count, Hamilton still has a cloud hanging over his future.
Phonak announced on Thursday that a follow-up test on the blood sample taken during the Tour of Spain had come back positive.
The International Cycling Union said it had handed Hamilton's file to the US Anti-Doping Agency in Colorado for further investigation.
If found guilty of blood doping, the 33-year-old rider could face a two-year ban from the sport.
In a statement published on the company's website, Phonak said it had decided to convene a panel of experts “in order to achieve clarity as to the medical... reliability of these new blood testing tools”.
“The team's goal is, and this is in the exact interest of Tyler Hamilton, that we have clarity in the end.”
Phonak said Hamilton would remain a member of the team but would not take part in races until the doping affair had been cleared up.
The two original tests on the cyclist suggested that Hamilton had undergone a blood transfusion - seen as a way of boosting an athlete’s performance by increasing his or her red blood cell count.
But Phonak, which has threatened to fire Hamilton if he fails to prove his innocence, has expressed doubts about the validity of the tests for blood doping, which were only introduced at the start of this season.
In a statement on Thursday, Hamilton repeated that he would prove his innocence.
“Hopefully with this scientific team, we can find a foolproof method to this test in order to prevent other athletes ending up in the same situation,” he said.
Hamilton joined Switzerland’s Phonak team last year. Company chairman Andy Rihs said at the time that he was delighted to have signed up one of cycling’s biggest names.
His new star paved the way for the Phonak team to join the Tour de France.
Hamilton is the second Phonak cyclist to become embroiled in a doping scandal this summer.
Last month Oscar Camenzind tested positive for the blood-boosting hormone EPO just before the Olympic Games.
Phonak tore up his contract immediately. Camenzind, who has since announced his retirement from professional cycling, was also suspended from competition for two years.
swissinfo with agencies
Tyler Hamilton was born on March 1, 1971, in Marblehead, Massachusetts.
He made his professional debut for Montgomery Belle in 1995.
He joined US Postal in 1996 before transferring in 2002 to Denmark's Team CSC.
His most notable victories have come in the Tour of Denmark in 1999 and the Dauphiné of 2000.
Hamilton has a two-year contract with Phonak.
Hamilton rode with Tour de France winner Lance Armstrong in the US Postal Service team for six years.
He left in 2001 to become the leader of Denmark's Team CSC, which won the team competition at last year’s Tour de France.
He finished fourth in the 2003 Tour de France, but pulled out of this year’s event after two weeks because of a back injury.