Pierre Bordry, the head of the French Anti-Doping Agency (AFLD) that tested Ricco, explained how the Saunier Duval leader avoided an anti-doping chaperone after the time trial on 8 July.

"When he knew he was going to be tested, he went off, and it is the escort who caught him up," Bordry said yesterday.

"He found himself blocked off. There was a traffic jam of cars and he could not get through the cars."

Bordry said AFLD staff had demanded an apology from Ricco.

"We asked him to apologise to the agency and the staff, which he did along with his team director," Bordry said. "We told him that, because of his behaviour, we were now going to test him after each stage. He put a target on himself."

After Ricco's positive test, Saunier Duval quit the race. Ricco and team-mate Leonardo Piepoli have since been fired.

Bordry has not ruled out further positive tests in the Tour de France but said he thinks the sport would be cleaner after the race finish in Paris.

"I believe cycling will be a cleaner sport after this Tour than it was at the start," he said before the start of the 16th stage in Cuneo.

"We're cleaning the sport, we're almost there but I can't rule out that there is some dirt left."

Three riders – Spaniards Manuel Beltran and Moises Duenas, along with Ricco – have failed dope tests for banned blood-booster EPO since the start in Brest earlier this month.

The AFLD had conducted blood tests before the start of the Tour, which revealed a number of odd parameters, then daily urine tests after stages and unannounced blood, urine and capillary tests in team hotels before and after stages.

"A number of rider profiles seem to indicate that they have stopped doping practices for fear of being caught," said Bordry.

He added that targeted testing was as much a deterrent as a means of repression.

Asked why Tour leader Frank Schleck of Luxembourg was twice controlled in his team's hotel in Cuneo on Sunday and Monday evening, Bordry said such moves were often tactical.

"A rider who's been tested one day does not necessarily expect to be tested again the next day," he said.

Patrice Clerc, president of Tour organisers ASO (Amaury Sport Organisation), said Bordry's message was reassuring and that he hoped the Tour would not be tarnished by more doping cases.

"The message that we're gradually cleaning the sport is getting through but another positive test affecting a leading rider could ruin our efforts and lead people to believe that this is a lost war," he said.