French authorities have opened a preliminary inquiry into doping allegations made against six-time Tour de France winner Lance Armstrong in a book published last summer.

Judicial officials confirmed the probe on Thursday but declined to give details, describing the case as confidential.

Prosecutor Philippe Drouet was not available for comment. Calls on Thursday by The Associated Press to Armstrong's press officer, Jogi Muller, were not immediately returned.

The investigation centres on a magistrate's interview with Armstrong's former British assistant, Emma O'Reilly.

Shortly before last year's Tour de France, O'Reilly made allegations in a book LA Confidential, The Secrets of Lance Armstrong. She claimed that Armstrong sent her on long road trips to pick up pills and dispose of used syringes.

According to Thursday's edition of Le Parisien newspaper, O'Reilly flew in from Liverpool, England, in July to confirm to French magistrates what she'd told authors David Walsh and Pierre Ballester in the book.

Le Parisien also said narcotics police questioned O'Reilly and other witnesses close to Armstrong.

Armstrong is suing Walsh and Ballester for defamation and is asking for 2 million euros ($A3.43 million) in damages.

Because of allegations made in the book, Armstrong's Texas-based insurance company has withheld a $US5 million ($A6.59 million) bonus owed to him after he won his sixth Tour last July.

In September, Armstrong's management agency - Capital Sports and Entertainment - said it had provided clean drug test results and released a statement saying "the baseless and mean-spirited doping allegations against Lance are not supported by any facts."

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