As leader of a yakuza – or Japanese mafia – gang, Goto amassed a fortune from prostitution, protection rackets and white-collar crime, while cultivating a reputation for extreme violence.

Tomorrow, his life will take a decidedly austere turn when he begins training at a temple in Kanagawa prefecture south of Tokyo, the Sankei Shimbun newspaper said today, citing police sources.

The 66-year-old, whose eponymous gang belonged to the powerful Yamaguchi-gumi crime syndicate, was expelled from the yakuza fraternity last October after a furious row with his bosses over his conduct.

Known as Japan's answer to John Gotti, the infamous mafia don, Goto reportedly upset his seniors amid media reports that he had invited several celebrities to join his lavish birthday celebrations last September.

Several months earlier he had attracted more unwanted publicity following revelations that he had offered information to the FBI in return for permission to enter the US for a life-saving liver transplant in 2001.

At an emergency meeting last October the Yamaguchi-gumi's bosses – minus their leader, Shinobu Tsukasa, who is serving a six-year prison term for illegal arms possession – expelled Goto, splitting his gang into rival factions.

According to the Sankei, Goto will formally join the priesthood on 8 April – considered to be Buddha's birthday in Japan – in a private ceremony.

The former gangster was quoted as describing the occasion as "solemn and meaningful, in which Buddha will make me his disciple and enable me to start a new life".

In his deal with the FBI, Goto reportedly gave up vital information about yakuza front companies, as well as the names of senior crime figures and the mob's links to North Korea.

Underworld experts have pointed out, however, that the bureau could have gleaned the same information from yakuza fanzines.

Goto's transplant was performed at UCLA medical centre in Los Angeles In the spring of 2001 by the respected surgeon Dr Ronald W Busuttil, using the liver of a 16-year-old boy who had died in a traffic accident.

The grateful don, who was suffering from liver disease, later donated $100,000 (£68,000) to the hospital, his generosity commemorated in a plaque that reads: "In grateful recognition of the Goto Research Fund established through the generosity of Mr Tadamasa Goto."

Jake Adelstein, a former crime reporter for the Yomiuri Shimbun newspaper, received death threats before he went public with the transplant story last spring, and has been living under police protection ever since.

When it was assigned to cultivate the Tokyo area in the late 1980s, the Goto-gumi stuck to what it knew best: drugs, human trafficking and extortion, before new anti-gang laws forced it to move in to more lucrative areas such as real estate and the stockmarket.

At the height of their powers, Goto's henchmen were capable of unspeakable acts of violence, including bulldozing businesses that refused to pay protection money and administering beatings to victims in front of their families, reports said.

A 1999 leaked police file noted that "in order to achieve his goals, [Goto] uses any and all means necessary or possible. He also uses a carrot-and-stick approach to keep his soldiers in line. His group is capable of extremely violent and aggressive acts".