A female restaurateur has taken on the might of Japan's most powerful Yakuza crime clan by suing its godfather for more than £120,000 she handed over in protection fees.

In what could be a landmark case, the woman's legal team is using an anti-organised crime law that states that the head of an organisation is responsible for the activities of its members. She is trying to reclaim the money extracted by the underlings of Kenichi Shinoda, sixth head, or kumichi, of the Yamaguchi-gumi.

It is alleged that the woman, who is reported to be under 24-hour police protection, was made to hand over 30,000-100,000 yen (£200-£650) at a time for 12 years by the Yamaguchi-gumi, which has 30,000 members.

Yakuza gangs are among the largest and best organised in the world, and are believed to extract millions in protection fees from businesses every year. The restaurant owner's lawyer, Kiyotaka Tanaka, said his client's lawsuit could severely weaken the Yamaguchi-gumi clan. "This woman showed great courage in standing up [to this injustice]," he said. "We would like this trial to become a curtailment for these activities in the future."

In the Nagoya area alone, it is estimated that the Yamaguchi-gumi collects a couple of hundred million yen monthly, a member of the Aichi prefectural police told the Tokyo Reporter. "Should this end, the Yamaguchi-gumi will take a strong hit."

The 98-year-old, Kobe-based Yamaguchi-gumi, is coming under increasing pressure from police and investigators.

Last week 29-year-old Yujiro Kobayashi and three others were detained for a second time over a 1bn-yen fraud that hit 450 people in Tokyo, in which the victim was called by a criminal pretending to be a relative in dire need of money.

Three people arrested in connection with the protection money case have denied involvement, while a fourth is refusing to speak.

it is believed that if the restaurateur's suit is successful, it could trigger a deluge of similar cases.

In 2010, the Kodo-kai, an affiliated gang, burnt down a hostess club in a dispute over a protection deal, and yakuza experts believe relatives of the victim may sue.

It is reported that Shinoda, 71, who has served time for murdering a rival with a samurai sword, recently issued a glossy magazine to 27,700 regular gang members entitled "let's march through these challenges", arguing for reform of the Yamaguchi-gumi's business practices.

Japan's National Police Agency claims the number of Yamaguchi-gumi members has fallen by 3,300 in the last 12 months.