The Filipino has refused to agree to the Olympic-style blood-testing terms set out by Mayweather’s promoters, sinking hopes that the contest - which would be one of the most lucrative in history - will go ahead.
Mayweather’s camp, represented by Golden Boy Promotions, has called for random blood and urine sampling before and after the fight as mandated by the US Anti-Doping Agency.
Manny Pacquiao fight in doubt
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Richard Schaefer, the chief executive of Golden Boy Promotions, said that Pacquiao’s promoters, Top Rank, had stated that he had agreed to have blood taken for testing before the initial media conference and after the fight, but would not agree to have blood drawn within 30 days of the bout.
The only hope is that the camps can somehow come to a surprise agreement today.
Promoter Bob Arum, who represents Pacquiao, said using such blood tests was unprecedented for professional fights in Las Vegas.
“As far as I’m concerned, the fight is off,” Arum said. “We don’t object to more extensive drug testing, even though it’s certainly not required in Nevada.
“What we’re saying is that the drug testing he is proposing is intrusive and would disturb Pacquiao’s training if it’s done within 30 days of his fight.”
Alex Ariza, Pacquiao’s LA-based strength and conditioning coach, alleged it was a ruse by Mayweather to allow him to pull out.
“He is desperate to protect his legacy, and could not face the prospect of possible defeat,” Ariza said.
“Even though there is $40 million (£25 million) on offer for him in this fight, Mayweather’s mindset means he may not be able to cope with the idea of having a loss on his record.”
Freddie Roach, Pacquiao’s trainer, disclosed privately to The Daily Telegraph recently that he had his doubts over whether Mayweather would go through with the contest, and was already looking at alternative opponents for his fighter.
Mayweather, meanwhile, has urged Pacquiao to agree to the testing.
“I understand Pacquiao not liking having his blood taken. Frankly, I don’t know anyone who really does,” he said. “But in a fight of this magnitude, I think it is our responsibility to subject ourselves to sportsmanship at the highest level.”
Pacquiao was to defend the World Boxing Organisation title he won in November by stopping Miguel Cotto in the 12th round in Las Vegas
for an unprecedented seventh title in seven different weight classes.
The showdown between Pacquiao (50-3-2) and unbeaten Mayweather (40-0) - to decide who is the best pound-for-pound in the world - was expected to be among boxing’s biggest revenue-producing fights.
Arum said alternative opponents for Pacquiao included Mexico’s Juan Manuel Márquez, American Paulie Malignaggi or unbeaten Israeli Yuri Foreman, the World Boxing Association super welterweight champion.