After the case involving the Chinese swimmers became public in April last year, an independent investigation by Swiss prosecutor Eric Cottier in July said Wada did not show bias and acted reasonably.

Tygart was again critical and called on Wada to conduct a “more thorough” audit by an investigator appointed by a neutral third party.

Tensions between the two organisations have since remained high.

In July, Wada said Usada would face a compliance review, while the International Olympic Committee (IOC) altered the hosting contract for the 2034 Winter Olympics to allow it to strip Salt Lake City of the Games if US authorities did not respect the “supreme authority” of Wada.

Last month, Usada said it “fully” supported the US government’s decision to withhold a payment of $3.6m (£2.8m) to Wada.

Wada said by dropping its lawsuit it was “putting this behind us and moving forward in collaboration with our stakeholders for the good of all athletes around the world”.

However, it also said it determined it is “futile to argue with somebody who is unwilling to accept clear evidence, whose only goal is to damage Wada and the global anti-doping system, and who has no desire to find a resolution”.

Tygart remained critical of Wada in response, calling its actions “retaliatory, wasteful and abusive”.

He added: “It’s time for those who value clean sport to step up and get Wada right, as athletes deserve a fair, robust global watchdog to protect their rights to fair competition.”

Usada said Wada has also dropped an ethics case against former US Office of National Drug Control Policy director Rahul Gupta, who represented the US on Wada’s executive board.