The WBC minimumweight champion, known in Thailand as Wanheng CPF, announced on his Facebook page on Sunday that he was retiring for health reasons.
He said he had thought thoroughly before making the decision.
“Nobody knows my body better than myself,” he wrote. “Everybody wants money but I prefer protecting my body. So I am quitting boxing.”
In his previous bout, the 34-year-old boxer, whose real name is Chayaphon Moonsri, scored a unanimous decision over South Africa’s Simphiwe Khonco in Chon Buri last October.
Piyarat said he had a talk with him yesterday.
“I talked to him on the phone and he admitted he did it because of stress during the COVID-19 pandemic,” he said.
Wanheng was scheduled to defend his title against the Philippines’ Marco Rementizo in California last month.
But the match, which would be Wanheng’s first outside his country, was postponed indefinitely because of the coronavirus outbreak.
His handlers have signed a five-bout contract with US-based Golden Boy Promotions for Wanheng to fight in America.
Piyarat said he wanted to arrange a warm-up fight for Wanheng but he could not bring in a foreign boxer due to travel restrictions.
Also, fighting against a fellow Thai would be improper, he said.
Piyarat said he planned for Wanheng to return to the ring in September.
Wanheng admitted he made the announcement without consulting anyone.
He said he would have a check-up and let the doctor decide if he could continue fighting.
Menayothin boxing gym chief Chaisit Kongkiatkong said the champion had complained he had a breathing problem during training.
“I can’t prevent him from quitting. Everything is up to him,” said Chaisit, who is also his personal manager.
Wanheng took the crown from Mexico’s Oswaldo Novoa with a ninth-round stoppage in November 2014.
Since then, he has won 18 consecutive bouts, including 12 title defences.
Wanheng is the longest-reigning active world champion with an unblemished record of 54 wins, with 18 knockouts.
He is currently one of Thailand’s only two world champions with the other being WBA minimumweight title-holder Knockout CPF.