The organisers of the Tour de France will refuse to let Chris Froome race in this year's event if his salbutamol case has not been resolved, it is understood.
The four-time Tour champion returned an adverse finding for the asthma drug salbutamol during his winning ride at last year's Vuelta a Espana.
Froome denies any wrongdoing and is continuing to race this season - as is his right under the World Anti-Doping Agency's rules - while his team of lawyers and scientists work on an explanation for the adverse sample, which contained twice the allowed concentration of the drug.
The Team Sky rider confirmed on Wednesday that his final warm-up race for his first big target of the season, the Giro d'Italia, will be the Tour of the Alps, a five-day race in Austria and Italy that starts on April 16.
The Giro d'Italia's organisers have already said they are powerless to stop the 32-year-old Brit from riding in their race and new International Cycling Union (UCI) president David Lappartient has confirmed that Froome's case will not be heard before the race starts on May 4.
But two senior cycling sources have told Press Association Sport that ASO, the French company that runs the Tour, has more discretion on who it registers for its event and has no intention of letting a rider with a potential anti-doping violation hanging over them to race.
ASO is understood to be confident that it could resist any legal challenge from Team Sky as it has clauses in its rules about safeguarding the image of the race.
This would be a bitter blow for Froome, who is chasing a fourth straight victory in cycling's most famous race and a record-equalling fifth win in total.
According to its rules, the UCI could also suspend Froome but Lappartient said at an event in Geneva on Wednesday that it did not want to do that.
"It's possible and it's true that we have this power," the Frenchman said.
"But for salbutamol, it's never been done, and we have to respect the rights of Chris Froome. It's not possible to have a specific treatment for him.
"And no other international federation has taken this decision for salbutamol. So if we were the only international federation to do this - and just for one rider - I think we would be in the wrong and could badly lose if it went to (the Court of Arbitration for Sport)."
Asked if the UCI would intervene to avoid the scenario of a race organiser having to make this choice, Lappartient said: "I think it's the UCI's job to deal with this matter - not the race organisers.
"But some of them are worrying about the consequences of this situation for their race and they wonder if they will have to try to refuse a rider.
"We hope that won't happen and this is resolved quickly, so the organisers don't have to do this. It's not their job, it's more a job for the UCI, WADA (the World Anti-Doping Agency), CAS (Court of Arbitration for Sport) and so on."
ASO declined to comment on the specifics of Froome's situation but told Press Association Sport it hoped for a "fast outcome" to his case.
The 2018 Tour starts in France's Vendee region on July 7 and finishes in Paris on July 29.