LAS VEGAS — A top anti-doping official says Floyd Mayweather Jr. and Manny Pacquiao face far more severe penalties for a prefight positive test for a banned substance than a $5 million fine unsuccessfully pushed by a Pacquiao adviser.

Travis Tygart, the chief executive of the United States Anti-Doping Agency, said a positive test would kill the May 2 fight, where the total purse likely will be more than $200 million. It also would subject the fighter who tested positive to a potentially career-ending ban of four years from competition.

"If there's a positive test prior to the fight, the penalty for the fighter who violates it is going to be a lot higher than $5 million," Tygart said Friday.

Both fighters signed agreements for the USADA to oversee drug testing for the May 2 fight that will be boxing's richest ever. Mayweather is expected to make at least $120 million and Pacquiao $80 million for the much anticipated bout.

Pacquiao adviser Michael Koncz had pushed for a $5 million penalty for a positive test, something the Mayweather camp rejected. The issue reportedly wasn't settled until the night before the fighters met at a Los Angeles press conference earlier this month, when contracts were signed.

Tygart said that if either fighter tests positive for a banned substance the results immediately would be given to the Nevada Athletic Commission and that the commission and the anti-doping agency would take disciplinary action together against him.

"They signed on to the sanctioning process that is clearly spelled out," Tygart said. "It's a contract that would be enforceable against them."

USADA abides by the World Anti-Doping Code, which spells out which substances are illegal and what the penalties are for testing positive. If either fighter tested positive for something like steroids or human growth hormone, they would face a ban of four years from competition that likely would be honored by most boxing commissions.

Drug testing was an issue when talks first began five years ago on the possible fight, with Mayweather's camp insisting that both blood and urine tests be done on each fighter. Mayweather on many occasions used Pacquiao's reluctance to do full testing as the main reason the fight wasn't done.

Pacquiao sued Mayweather for defamation for saying he used performance-enhancing drugs, a case that was later settled out of court.