The mandatory questionnaire that the Osaka Municipal Government issued in February 2012 to ask employees about their union activities constituted an unfair and illegal labor practice, a prefectural labor panel ruled Monday.

The Osaka Prefectural Labor Relations Commission ordered the city to submit a written promise to the unions not to repeat such a survey.

Osaka Mayor Toru Hashimoto expressed regret and said he would have to apologize to the unions, while union leaders said the ruling proved the mayor's actions were wrong.

Hashimoto issued the survey to about 32,000 employees so he could determine whether they supported his predecessor in the 2011 election. He threatened punishment for those who gave inaccurate replies.

The commission said the survey was conducted by the mayor in a forceful manner because he opposed the employees' union activities.

In response to a motion filed by the union against the survey, the commission asked the city in February last year to suspend the survey. Lawyer Shuya Nomura, who was responsible for the survey as a special adviser to the city, discarded the responses without opening them.