While International Cycling Union regulations state that all ProTour teams are assured a start in the three big stage races, Vuelta (Spain), Giro (Italy) and Tour organisers are not part of the UCI ProTour.
"There will be discussions but no one can be sure yet of being at the start of the Tour," Prudhomme said at the unveiling of the route for the 95th race. "There should be 20 teams, more or less."
Prudhomme said that simply having a ProTour license was no guarantee of participation in the Tour de France.
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"No one will tell us who can participate in the Tour de France," said Patrice Clerc, president of race owners Amaury Sport Organisation, emphasising the point.
All riders wishing to participate should be in possession of the "biological passport" currently being established by the UCI and World Anti-Doping Agency, Prudhomme added.
WADA and the UCI plan to collect blood samples from all professional riders in 2008 to create a medical profile that would then be compared to the data registered in doping tests.
Clerc said he was proud that cycling was an example to other sports in the fight against doping.
"Cycling is now ahead of any other sport," he said, adding that the big Tour organisers and the UCI, who have been at odds since 2004 over the ProTour, should renew dialogue on the issue.
"We will end this crisis if we all sit together around the same table. All families in cycling must work together. If we manage to do so, we could start the next Tour united again," he said.
Cycling has been hit by an unprecedented number of doping cases this season, several during the Tour.
The Astana team were urged to leave this year's Tour by the organisers after Kazakh Alexander Vinokourov was kicked out following a positive test for blood doping. This year's winner, Spaniard Alberto Contador, has now joined Astana.
Last month, American Floyd Landis was stripped of his 2006 Tour title after being found guilty of doping.
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