Hamilton's future remains uncertain

The IOC said that the result of the American's B, or second, test was "non-conclusive" because the sample had been destroyed by being deep-frozen.

Earlier, Hamilton's Phonak team stated the B test on a blood sample he gave at the Tour of Spain had tested positive.

Phonak said they are to investigate the discrepancies between the tests.

Hamilton had been told he gave a positive A, or first, test for a blood transfusion in Athens on 18 August.

He then failed another test, conducted by cycling's govening body the UCI, after winning a time trial in the Tour of Spain on 11 September.

BLOOD DOPING EXPLAINED What is it? The administration of red blood cells to increase the blood's oxygen-carrying capacity Examples include: Injecting someone else's red blood cells; removing own blood and returning it once body has compensated by making more blood Why would athletes do it? The better the oxygen-carrying capacity of the blood, the better an athlete's endurance Side effects can include: Blood clots, overload of circulatory system, kidney damage, transmission of infectious diseases Chances of being caught: A new test can only detect the practice if the blood comes from a donor

Hamilton is the first cyclist to be found to have tested positive for blood doping - when athletes inject blood from another person in order to increase red blood cells and improve stamina.

Phonak said: "The new method (of blood testing) is based on probability and interpretation measurements (so) uncertainty will remain in this examination."

If the B sample from Hamilton's Athens test had been positive, he would have almost certainly lost his gold medal.

Arnie Ljungqvist, head of the IOC's medical commission, said it had been a mistake to deep-freeze Hamilton's B sample.

"The blood sample was unfortunately destroyed. It should not have been deep frozen (in the laboratory)."

Hamilton, 33, has maintained he is is "100%" innocent of the charges.

He could be banned for two years, effectively ending his career, if the UCI finds him guilty of doping offences in the Tour of Spain.