The 32-year-old, who finished third in last year's race, tested positive for the diuretic Xipamide on 14 July.

Schleck, 12th in the Tour's overall standings, "categorically denies" taking any banned substance and insisted that his 'B' sample be tested.

"If this analysis confirms the initial result, I will argue that I have been the victim of poisoning," he said after being dropped.

Schleck, the older brother of 2010 winner Andy, was nine minutes and 45 seconds behind leader Bradley Wiggins who resumed stage 16 today.

His RadioShack-Nissan team said they withdrew him from the race so he could prepare a defence.

Schleck voluntarily attended Pau police station yesterday – the last rest day in the Tour – for questioning.

Cycling's world governing body, the Union Cycliste Internationale (UCI), informed Schleck of his failed test earlier in the day and requested his removal from the Tour, even though its anti-doping rules do not require a provisional suspension.

There were uniformed and plain clothes police at the RadioShack-Nissan hotel, but team spokesman Philippe Maertens denied they had carried out a search of the rooms and were merely there to keep the peace and deal with the media.

Schleck denied wrongdoing and immediately exercised his right to demand analysis of his 'B' sample.

He said in a statement: "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."

A RadioShack-Nissan statement added: "The reason for the presence of Xipamide in the urine sample of Mr Schleck is unclear to the team.

"The team isn't able to explain the adverse findings at this point."