The inquiry began in November last year when Tour de France officials received an anonymous letter.

The letter suggested a French television crew witnessed suspicious behaviour around the US Postal Team during the 2000 Tour de France.

Armstrong has repeatedly denied taking banned substances, but he has been dogged by suspicions of doping - particularly in France.

Francois Franchi, assistant to the Paris prosecutor, said: "We have no certainty about the use of a doping substance.

"We do not know whether something was taken. All the tests are negative."

Armstrong and nine team-mates were summoned to appear last month before investigators seeking additional information.

Franchi did not say when he expected the inquiry to finish.