Sprinter Chambers and cyclist Miller will be cleared to compete in this year's Games when the Court of Arbitration (CAS) overrules a British Olympic life ban on doping offenders, a source with knowledge of the ruling told Reuters on Sunday.
CAS is to announce later on Monday that the British Olympic Association (BOA) lifetime ban does not comply with WADA's global code, which provides for a two-year suspension for first-time offenders. That decision from the Lausanne-based independent court is due to be released at 1400 GMT.
USADA CEO Travis Tygart said: "The authority of WADA has been recognised ... and it is a good day for those who believe in the ability of WADA to put in rules to protect, in a fair manner, the rights of clean athletes on a level playing field.
"When you set the rules and the world agrees to those rules, that is what all athletes rely on. It would be an unlevel playing field for one entity, or one organisation, to attempt to change those rules."
Former world indoor 60 metres champion Chambers was banned from athletics for two years in 2003 after testing positive for the designer steroid THG. Millar was also banned for two years after admitting taking the blood booster EPO.
WADA's code, which came into force in 2004, harmonised rules across sport to bring in a maximum ban of two years for athletes testing positive for performance-enhancing drugs for the first time.
A BOA bylaw meant that any athlete given a doping ban of six months or more would not be selected for the British Olympic team, effectively meaning an Olympic life ban.
WADA, however, see that bylaw as an additional sanction and consider it non-compliant with the global code, of which the BOA is a signatory.
"Assuming that's the case, it's obviously the right outcome for all of those who rely on harmonization and certainty in the anti-doping movement to protect clean athletes and the integrity of the competition," Tygart said of the CAS ruling.
"We are in the middle now of the third review (of the WADA code) and the world has the opportunity to provide input to what those rules should be. And certainly there is discussion of what is the appropriate sanction for intentional cheats.
"We (USADA) certainly support, as do clean athletes, a strong, strict sanction in intentional cheating cases that could include up to lifetime (bans).
"But importantly, once the rules are set, it is only fair that everyone who has agreed to those rules abides by them and does not unilaterally attempt to undercut those rules as was attempted by the BOA in this case."
Tygart said consideration of specifics must still play a part when dealing with the issue of drugs in sport.
"In the intentional cases, like when Marion Jones cheated at the Olympics, and went on through that cheating to win five Olympic medals, and later had to return them and served a penalty, that penalty should also include not being able to compete in future Olympic Games," he said.
"But does that mean (swimmer) Jessica Hardy, who had a contaminated supplement according to a CAS panel, should never compete in Olympic Games? I think we have to appreciate that in certain cases people are entitled to rehabilitation and redemption.
"And frankly those athletes can serve as role models for future generations. "