A car driven by the father of Tour de France stars Andy and Frank Schleck (CSC-Saxo Bank) was searched by French customs during the 18th stage, which began in Bourg d'Oisans on Thursday.

An official source in Grenoble later said that "no doping products or banned substances" were found during the long search, which took place at Vizille in the Isere region of the Alps.

Andy Schleck suggested "jealousy" may have been behind the search.

When told of the news, Andy — who is wearing the race's white jersey for the best placed rider aged 25 years or under — said he was a little shocked, but unconcerned.

"I heard that the customs stopped my dad and a family friend who is often traveling with us, but it's not a problem for me," said Schleck, whose older brother Frank is second overall in the race at 1:24 behind their CSC teammate Carlos Sastre of Spain.

"It's not the first time we've seen something like that happening at the Tour. I'm a bit shocked, but it's not a problem. I don't know what was behind it, maybe someone's jealous or something."\

Johnny Schleck, a former professional cyclist who raced with Belgian legend Eddy Merckx, was stopped along with another vehicle as both cars were driving along with the advertising procession which precedes the stage itself.

Several journalists and photographers witnessed the scene, at the 30km mark of the 196.5km stage.

Customs chiefs confirmed that two Luxembourg-registered cars - one of which was being driven by a journalist who was later allowed to go - were stopped and searched.

"One of the vehicles was taken out of the public's view for a more thorough search," said a customs official.

According to high-ranking customs officials in Paris, the operation was carried out "as part of a program to target banned substances on the Tour de France."

In a television interview Johnny Schleck said he was stunned that people could suspect his sons of doping.

"My sons race cleanly. With all the doping controls they are subjected to how would it be possible for them to be otherwise?" he said.

In 2002 third place Tour finisher Raimondas Rumsas of Lithuania was suspected of doping after his wife Edita was discovered to have doping substances in her possession during a customs search at the French border.

Rumsas completed the Tour without testing positive for any banned substances. A stash of illegal products, including EPO (erythropoietin) and testosterone were found in his wife's car.

CSC team manager Bjarne Riis said the search was of little concern to the team or to the Schleck brothers.

"The police do their work and we do ours," Riis said. "We have nothing to fear. We’re here to get good results and do good things in the race. We will stay calm. "