Olympic time-trial champion Tyler Hamilton, captain of Switzerland’s Phonak cycling team, has denied any wrongdoing after reportedly failing two doping tests in just over a month.
Phonak disclosed on Tuesday that Hamilton was being investigated for possible blood doping.
If a further test confirms preliminary results, Hamilton will become the first victim of new controls on performance-enhancing blood transfusions.
But at a press conference near Zurich on Tuesday evening, Hamilton said he was "100 per cent innocent".
"I will fight this until I have not a single euro left in my pocket," he added.
The United States-born cyclist apparently failed tests at last month’s Olympic Games in Athens and at the Vuelta cycling tour currently underway in Spain.
According to Phonak spokesman Georges Lüdinger, both tests showed the “presence of a mixed red blood cell population, indication of a homologous blood transfusion”.
A follow-up test designed to confirm the preliminary results is scheduled to take place this week.
Lüdinger said that Hamilton had denied undergoing a transfusion – seen as a way of boosting an athlete’s performance by increasing his or her red blood cell count.
“Tyler told us he did nothing,” he said.
The Lausanne-based International Olympic Committee (IOC) said it was waiting for more evidence.
“As with all doping procedures, while a process is underway, we can’t go into details,” said IOC spokeswoman Giselle Davies.
IOC medical director Patrick Schamasch said he could “not confirm or deny anything” before all the test results had been received.
If found guilty of a doping violation at the Olympics, Hamilton would be stripped of the gold medal he won in the men’s time trial.
Hamilton pulled out of the Spanish Vuelta after the 12th stage last Thursday, citing stomach pains.
He rode with Tour de France winner Lance Armstrong in the US Postal Service team for six years.
Hamilton left in 2001 to become the leader of Team CSC, a Danish club that 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.
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 and at the same time helped the company to market its hearing aid products in the US.
The 33-year-old 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 Danish team CSC.
His most notable victories came in the Tour of Denmark in 1999 and the Dauphiné of 2000.
Hamilton has a two-year contract with Phonak.