VIZILLE, France (Reuters) - French customs stopped and searched a car driven by Johnny Schleck, father of riders Frank and Andy, during Thursday's Tour de France 18th stage.

"We were looking for doping substances but the check was negative so we let him (Johnny Schleck) go," a customs spokesman told Reuters by telephone.

"Why would I have banned substances?" Johnny Schleck told RTL radio. "My sons are clean. With all these tests they have been undergoing, how would they not be clean?"

Frank Schleck, who lost the race leader's yellow jersey on Wednesday, is second overall one minute and 24 seconds behind Spanish CSC team mate Carlos Sastre.

He declined to comment when asked for a reaction at the end of the 196.5km ride from Bourg d'Oisans to St Etienne.

Customs officers had stopped the car driven by Johnny Schleck, who drives guests around for one of the Tour sponsors, 35 km into the stage. He was asked to step out of the car at a roundabout and was searched by a police officer.

He was then asked to drive the car towards the other side of the roundabout, where the vehicle was searched by customs officers for 30 minutes.

The car later continued on the stage course behind a customs vehicle towards Grenoble.

(Additional reporting by Clement Guillou)

(Reporting by Julien Pretot, editing by Ken Ferris)