Lawyers representing 1,789 customers of educational services company Benesse filed a 100 million yen suit for damages against the firm on Thursday over a data leak that affected more than 20 million people last year.

One of the lawyers told reporters outside the Tokyo District Court that the massive data leak of confidential information such as names and addresses had caused great anxiety among many people, TBS reported.

The lawyer said another 1,000 plaintiffs are expected to join the damages suit by mid-February.

Masaomi Matsuzaki, a systems engineer, was arrested last July for stealing massive amounts of personal data -- including names, addresses, phone numbers and birthdays -- multiple times, which he then sold for about 2.5 million yen.

The magnitude of the leak prompted angry calls for an explanation from Benesse, one of the country's best-known companies. The firm, whose holdings include the Berlitz language education brand, offers services ranging from teaching babies how to use toys to English instruction.

Benesse confirmed the leak of personal data of at least 7.6 million people, but said the problem could ultimately affect more than 20 million.

However, other details such as credit card numbers, banking data and student performance evaluations were safe, it said.

Matsuzaki was a systems engineer at Benesse affiliate Synform Co when he copied personal details of customers who had supplied the information to the educational services company, according to police. They said a minimum of 10 million were affected.

The stolen data was sold to at least 10 list brokers, including JustSystems Corp which used it to send ads to Benesse customers.

The theft was discovered when people started receiving phone calls and advertising letters from Benesse rivals that should not have had the information, authorities said.

The Ministry of Economy, Trade and Industry (METI) reprimanded Benesse and ordered the company to improve its data protection systems and strengthen oversight.

© Japan Today