Japanese Transport Minister Keiichi Ishii said on Friday that unauthorized technicians had been found certifying vehicles at five Nissan plants that the ministry has been inspecting.

The unauthorized technicians included contract workers, Ishii told a news conference.

"It's extremely regrettable, causing anxiety for users and shaking the foundation of the certification system," he said.

Nissan has decided to recall all 1.2 million new passenger cars it sold in Japan over the past three years after discovering final vehicle inspections were not performed by authorized technicians.