The labor ministry said Friday its officials have been aware of an improper method used for wage surveys for more than a decade, raising the possibility of a systematic cover-up.

The fresh revelation could deal a blow to Prime Minister Shinzo Abe's administration, which is already under fire for another data scandal involving the ministry.

The Health, Labor and Welfare Ministry admitted that regional branch officials were supposed to visit companies in person for the wage structure survey but that in most cases, they sent questionnaires by mail.