Ullrich and Basso were both caught up Operation Puerto

Race director Christian Prudhomme says those under suspicion should not be allowed to compete in the near future.

He said: "Cycling can't afford to let riders in the case enter the tour if they're not cleared of suspicion."

More than 50 riders were named in the high-profile case, which was closed after the judge ruled no offences were committed under Spanish Law.

Prudhomme has already contacted the Tour teams involved in a bid to win their support.

The Tour director told French newspaper L'Equipe: "I have started discussing the subject with several team managers."

The Spanish Cycling Federation launched "Operation Puerto" last year.

It centred on links between riders and a Spanish doctor, Eufemiano Fuentes, after anabolic steroids, frozen blood and blood transfusion equipment were discovered in raids by Spanish police.

Earlier this month, a DNA sample taken from former Tour de France champion Jan Ullrich was matched to blood bags seized by the police.

The 1997 winner was withdrawn from last year's Tour by his T-Mobile team on the eve of the prologue after being named in the Puerto case.

The rider has since retired but other riders implicated in the investigation, including Discovery Team leader Ivan Basso of Italy, are still competing.