"The relationship between USADA and UFC became untenable given the statements made by UFC leaders and others questioning USADA's principled stance that McGregor not be allowed to fight without being in the testing pool for at least six months," USADA CEO Travis Tygart said in a release.
The UFC and McGregor did not immediately respond to a request for comment.
The USADA said meetings over a contract renewal broke down on Monday when, after a productive meeting in May, the UFC did an "about-face" and informed the agency it was going in a different direction.
"We are disappointed for UFC athletes, who are independent contractors who rely on our independent, gold-standard global program to protect their rights to a clean, safe, and fair octagon," Tygart said.
Reporting by Rory Carroll in Los Angeles; editing by Grant McCoolOur Standards: The Thomson Reuters Trust Principles.
Persons:
Anthony Joshua, Robert Helenius, Conor McGregor, Andrew Couldridge, McGregor, USADA, Travis Tygart, Tygart, Rory Carroll, Grant McCool
Organizations:
O2, UFC, U.S, Doping Agency, Thomson
Locations:
London, Britain, Los Angeles