The WBO ordered a review of the scoring of the July 2 Brisbane bout at the request of the Philippines' Games and Amusements Board.
The WBO ordered a review of the scoring of the July 2 Brisbane bout at the request of the Philippines' Games and Amusements Board, which criticised the judges and the referee.
The WBO set up a panel of independent and anonymous judges to watch the bout without sound and determine who won each round. On Tuesday it said the judges found Horn won seven rounds to Pacquiao's five.
"We have seen worse judgments in the past where judges manipulated results. Nothing surprises me now," Pacquiao said in a statement late Tuesday.
He did not name the judges nor mention the specific fights in his allegations.
"Let the people judge for themselves. People saw what happened," said the 38-year-old, who had won world titles in an unprecedented eight weight divisions.
The undefeated but unheralded Horn, 29, had stunned the Filipino legend with his ultra-aggressiveness to earn a unanimous decision.
But the Filipino's camp alleged the referee let the Australian get away with illegal tactics.
The WBO ordered the review while stressing it did not have the power to reverse a decision unless fraud or law violations were proven.
Pacquiao had endorsed the call for a review issued by the Filipino sports regulatory, which lashed out at the "unfair decision and officiating" of the fight.
In a statement sent to AFP on Wednesday, the Filipino regulator thanked the WBO for looking into the fight but blamed American referee Mark Nelson for the results.
"No matter what review they do, it will be hard to change the result as the referee didn't call it close," said board chairman Abraham Kahlil Mitra.
"The judges' scores would have greatly changed if the referee did his job properly," Mitra alleged.
"He (referee) didn't give (Senator) Manny Pacquiao the respect and fair protection that is due to a people's champ," Mitra said, referring to Pacquiao's elected post as a Philippine senator.
Horn, a former schoolteacher written off before the bout by most observers, welcomed on Tuesday the ruling as "evidence" of his victory.
"It's definitely nice to have it finally put on paper," said Horn, who had been keen for a rematch."
In the twilight of a 22-year pro career, Pacquiao had initially called for a rematch, but later said he would also "think hard" about retiring.
Pacquiao briefly quit boxing last year to pursue his long-held political ambitions and was elected senator. But he quickly made a successful comeback against Jessie Vargas in November, saying he still felt like a youngster.
Pacquiao's loss to Horn had prompted calls that he retire for good.