The Yomiuri Shimbun has confirmed that anyone was able to view the personal schedule data in question, which was posted on nine user calendars.

Earlier this month, Google stopped the public calendar search function of the service, which enabled users to search other users' calendars, without providing users in Japan with an explanation.

Even now, however, the calendars of users can be viewed by other users if they know the relevant calendar address.

The free calendar service can be accessed by personal computer or cell phone just by typing in a user ID. The calendar can be used as a personal memorandum or by a group of specified users.

On the initial settings page, the options for sharing the information with selected users and making all information public are close to each other.

Some users have been confused about the difference between the two options, with some mistakenly thinking they have to check both boxes to be able to use the calendar on more than one device or to share the calendar with friends.

However, once a user has chosen "Share all information on this calendar," it makes his or her calendar available to all users.

A 39-year-old surgeon who works at a hospital in Tokushima Prefecture mistakenly disclosed on his personalised Google Calendar from April last year about 150 items of information, including the names of patients and their conditions.

In one case, the information contained a patient's name and indicated that the patient had been operated on to fit a colostomy bag.

The hospital explained that the surgeon probably was not aware the calendar was viewable to other users because he thought it was personalised. Apparently the hospital has yet to decide how to explain the information leak to patients.

A lawyer in his 30s unwittingly disclosed his schedule, which included the names of clients, appointment dates and court schedules. Some of the disclosed data was sufficient to identify the individuals involved.

"I meant to share the calendar only within our office," said the lawyer, who works at a law firm in the Tohoku region. "Putting information up on the Net is dangerous."

Other cases include a company in Kyushu that unwittingly disclosed the date it was going to pay out bonuses, and a nail salon in Tokyo that unintentionally revealed a record of internal memos, including what to do with fees when customers expressed dissatisfaction with their service.

"We don't have any information on specific personal information leaks," Yoshito Funabashi, the public relations department director of Google Japan, said after the company removed the public calendar search function.

While a brief notice appeared on Google's English-language site, the company has not explained to Japanese users why the change was made.

Since the calendars are still viewable by typing in the relevant addresses, users who mistakenly set up their calendars to be shared with other users are still at risk of disclosing personal information.

Funabashi described the removal of the public calendar search function as "an improvement to enhance convenience."