Beltran was one Lance Armstrong's key team-mates
Manuel Beltran is being questioned by French police after a positive test for the banned blood booster erythropoietin (EPO) at the 2008 Tour de France.
After an anomaly in Beltran's pre-Tour blood test, the Spaniard was targeted by France's Anti-Doping Agency (AFLD).
And the 37-year-old Liquigas rider was found to have EPO in his urine 'A' sample taken after the first stage from Brest to Plumelec on Saturday 5 July.
"There are not just traces of EPO, there is EPO," said the head of AFLD.
"The rider concerned has been informed of the results of the analysis of the tests we took and has been informed of his rights," said Pierre Bordry, head of the AFLD.
Beltran has the right to ask for a second urine sample to be tested, but has been suspended and withdrawn from the race by his team manager Roberto Amadio.
And after French police searched his hotel room, Beltran was taken away.
"The police have taken Manuel away for questioning," said a Liquigas spokesman. "He was not sharing the room with any other team-mates. It was only his room that was searched."
Liquigas are likely to be allowed to remain in the race, if they manage to convince Tour organisers that Beltran's is an isolated case.
For us if this result is confirmed it is an isolated case
Liquigas team manager, Roberto Amadio
"Our first decision is to suspend Manuel," said team manager Amadio.
"He has told us he has done nothing wrong and he wants to have a counter-analysis done, so until then all we can do is to suspend him.
"But if that also tests positive, his contract with the team will be terminated."
Amadio added: "Our first reaction to this news is one of surprise.
"For us it's a very bad situation, for the whole team, including the riders who must start the race tomorrow.
"We have an active anti-doping programme in place, so for us if this result is confirmed it is an isolated case."
Last year the entire Cofidis team were expelled when Cristian Moreni tested positive for testosterone.
And the Astana team was also ejected from last year's race when Alexandre Vinokourov tested positive for blood doping.
A professional since 1995, Beltran began his career with the Mapei team, and also rode for Banesto and Team Coast, before helping Lance Armstrong to the last three of his seven Tour de France wins with the US Postal and Discovery teams.
Beltran is the fourth former Armstrong team-mate to test positive for doping; the others were Floyd Landis, Tyler Hamilton and Roberto Heras.
This year marks the 10th anniversary of the 'Festina Affair', when organised doping was uncovered in the then France-based team, leading to many of the current anti-doping systems.