Kobe Steel, a key supplier to the automakers and other heavy industries, has admitted it shipped products with falsified inspection certificates for at least a year.
The certificates are meant to confirm the product in question meets specifications agreed to by both Kobe Steel and the customer.
At present, Kobe says around 19,300 tonnes of rolled aluminium and extrusions, 2200 tonnes of copper products, and 19,400 units of aluminium castings and forgings were shipped with faked certificates.
With these items potentially used in load-bearing or structural components, their uncertain composition and strength raises a number of safety questions.
The company has begun speaking to affected customers individually, although it has yet to name any publicly. Kobe is working with these clients to carry out "technical verifications ... on the impact of the non-conforming products on quality (including safety) of the end products".
Kobe claims: "Verification and inspection to date have not recognised specific problems casting doubts on the safety of the non-conforming products.
"In the event that doubts arise on the safety of the nonconforming products, the Company will quickly take appropriate action."
According to The New York Times, Honda, Mazda, Mitsubishi, Nissan, Subaru, Suzuki and Toyota, as well as airplane manufacturers Boeing and Mitsubishi Heavy Industries have said they are investigating whether they have used any of the affected products.
Kobe says the falsified certificates "came to light following self-inspections and emergency quality audits of the compliance status of contracts executed as to products shipped over the past year".
An internal investigation is under way to discover how and why certificates were faked, as well as find any other instances of falsified certificates. An external law firm has been engaged to assist Kobe Steel with the investigation.
In a statement announcing its discovery of doctored certifications, Kobe Steel said: "Causing this serious matter has brought overwhelming shame to the Company.
"The Company deeply regrets this incident and sincerely apologizes for the enormous worry and trouble this incident has caused to its customers and other related parties. The Company will report again as further progress of the investigation is made."