Japanese Transport Minister Keiichi Ishii says unauthorised technicians have been found certifying vehicles at five Nissan Motor Co plants.
The unauthorised technicians included contract workers, Ishii told a news conference on Friday.
"It's extremely regrettable, causing anxiety for users and shaking the foundation of the certification system," he said.
The ministry said it has carried out spot inspections at all of Nissan's six assembly plants in Japan and found stamps of certified technicians were used at five plants on documents to sign off final vehicle checks conducted by non-certified technicians.
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 authorised technicians.
Ishii said Nissan was expected to file the recall with the ministry on Friday afternoon.