Many Pacquiao, the WBO world welterweight champion, has filed a lawsuit against Floyd Mayweather Jnr and the Golden Boy Promotions chief executive, Oscar de la Hoya, over claims he has used performance-enhancing drugs.
The Filipino fighter, a six-weight world champion, withdrew from a potentially hugely lucrative bout against Mayweather in March after the American's camp demanded Olympic-style drug-testing procedures be put in place in the lead-up to the fight.
The lawsuit launched by Pacquiao in a Las Vegas district court names Mayweather and his father, Floyd Snr, De la Hoya and fellow Golden Boy executive Richard Schaefer.
The lawsuit claims that the Mayweathers, De La Hoya and Schaefer knew that Pacquiao had never taken performance-enhancing drugs, knew their statements were false, but made them anyway out of "ill will, spite, malice, revenge, and envy".
Daniel Petrocelli, a lawyer acting for Pacquiao, said: "Many Pacquiao's achievements come from God-given talent and an indefatigable work ethic – not steroids.
"He cannot and will not allow others to deliberately misrepresent his years of hard work and tarnish his reputation."
The Mayweather camp insisted on both fighters making themselves available for testing before the projected 13 March fight, in accordance with US Anti-Doping Agency regulations. Pacquiao refused to submit to blood testing in the 30 days leading up to the fight, saying he would only give blood before the initial media conference and immediately after the bout, and spoke last week of starting legal proceedings against the Mayweather camp.
The US fighter's team tried to defuse that issue in a statement last week. It read: "Let it be very clear that nobody from Team Mayweather or Golden Boy Promotions is accusing Pacquiao of anything. But the reality seems to be that, for whatever reason, Pacquiao does not want to participate in random blood-testing, which has already been deemed a harmless procedure that many current athletes are subjected to, prior to and during competition."
However, Pacquiao hit back. "Enough is enough," he said. "These people, Mayweather Snr and Jnr, and Golden Boy Promotions, think it is a joke and a right to accuse someone wrongly of using steroids or other performance-enhancing drugs.
"I maintain and assure everyone that I have not used any form or kind of steroids and that my way to the top is a result of hard work, hard work, hard work and a lot of blood spilt from my past battles in the ring, not outside of it.
"I have no idea what steroids look like and my fear in God has kept me safe and victorious through all these years," added the 31-year-old Filipino, who has never tested positive.
Pacquiao's promoter, Bob Arum, is believed to have lined up Paul Malignaggi as an alternative opponent in March and the law suit against Mayweather makes that prospect more likely.