Allegations that athletes are using performance enhancing drugs ... and state-sponsored doping schemes ... will hang over the upcoming Paris Olympics.

Both China and the International Olympic Committee have lodged fierce complaints against American investigators who are looking into 23 Chinese swimmers who tested positive for banned drugs ahead of the 2021 Tokyo Games.

The IOC has even threatened to take the 2024 Olympics from Los Angeles and the 2034 Winter Games from Salt Lake City if the U.S. doesn't stop its investigation.  

We talked with Steven Bank ... he teaches international sports law at UCLA.  

He tells us this doping dispute stems from an American law aimed at discouraging the use of performance enhancing drugs.

Bank says there is not much trust or confidence that the World Anti Doping Agency ... or WADA ... is really serious about creating drug-free competition at the Olympics.

Although the threats to take the Olympics from American cities sound serious ... Bank doubts it will come to that.

Several American swimmers in Paris have said they have little faith that the competitions will be clean.