Frank Schleck has requested the analysis of his B sample while insisting he does not know why he tested positive for a banned substance which resulted in his withdrawal from the Tour de France.

The 32-year-old tested positive for banned diuretic Xipamide on July 14, cycling's world governing body.

Schleck was immediately withdrawn from the remainder of the Tour, beginning with the 197-kilometre 16th stage from Pau to Bagneres-de-Luchon, but his team, RadioShack-Nissan-Trek, remained in the race.

Schleck was 12th overall, nine minutes and 45 seconds behind leader Bradley Wiggins (Team Sky), following the 15th stage and has the right to attend the analysis of his B sample within four days.

It is a right he intends to exercise.

In a statement in French, distributed online by Luxembourg media outlet RTL, Schleck said: "I categorically deny taking any banned substance.

"I have no explanation for the test result and therefore insist that the B sample be tested which is my right. 

"If this analysis confirms the initial result, I will argue that I have been the victim of poisoning."

RadioShack-Nissan-Trek spokesman Philippe Maertens last night confirmed Schleck had voluntarily gone to the French police on receipt of the news for questioning.

Maertens said Schleck had gone to the police station because he knew the authorities would be seeking to speak to him.

Schleck left the police station in Pau late last night.

There were uniformed and plain clothes officers at the hotel, but Maertens insisted rooms had not been searched.

The positive test came after Saturday's Bastille Day stage to Cap d'Agde.

The news came on the Tour's second rest day, with five days of racing remaining before Sunday's conclusion in Paris.

The Luxembourg rider is the elder brother of Andy, who won the 2010 Tour after original champion Alberto Contador was stripped of his title for a doping offence. Andy Schleck missed this year's Tour due to injury.

Schleck has four times finished in the top 10 of the Tour and won two stages.

In 2008 he wore the race leader's yellow jersey for two days before finishing sixth.

It was not the first trouble for RadioShack-Nissan-Trek this season.

Team manager Johan Bruyneel opted not to attend the Tour due to his link with the United States Anti-doping Agency investigation into the United States Postal Service team, with seven-time winner Lance Armstrong a fellow defendant. Both Armstrong and Bruyneel refute all allegations of wrongdoing.

The RadioShack-Nissan-Trek team was formed this year from the amalgamation of Armstrong's former RadioShack team and the Luxembourg-backed Leopard Trek team.