Lists containing personal information related to around 103,000 people, including public health insurance card numbers, have been leaked, with some of the information sold off in what could be Japan's largest ever case of data theft.

An official at the Health, Labor and Welfare Ministry said lists compiled by hospitals and pharmacies may have been leaked, and that the ministry has begun an investigation.

The revelation comes amid growing concern over the government's handling of personal information ahead of the start next month of the My Number social security and tax number scheme, in which 12-digit ID numbers have been allocated to all residents of Japan.