Japanese officials on Thursday started an on-site inspection at the headquarters of Toyota subsidiary Daihatsu, the transport ministry said, after an investigation found it had rigged safety tests and was forced to suspend all shipments of its vehicles.

The independent probe found the malpractice dated back to 1989 and led Toyota to express its "sincere apologies" and pledge to carry out "a fundamental reform".

The panel was set up earlier this year to probe a safety scandal that emerged in April.

The investigation "found new irregularities in 174 items within 25 test categories" in addition to wrongdoing previously detected in April and May involving door parts and side-collision tests, Toyota said after the report was released.

On Wednesday, Daihatsu submitted a report to the transport ministry — having already provided one in May — saying new irregularities had been detected in an internal probe, and announced the suspensions.

"We began on-site inspection to find out if the report submitted by Daihatsu (on Wednesday) is true and if there is any other wrongdoing," transport ministry official Nobuhito Kiuchi told AFP.

"Before issuing administrative orders (as punishment), we have to find out facts around the issue," he said, noting that the on-site inspection will continue until at least early next year. 

Footage from Japanese broadcasters showed more than a dozen officials entering Daihatsu's Ikeda headquarters in Osaka prefecture.

Founded in 1907 to manufacture internal combustion engines, Daihatsu launched its first three-wheeled vehicle in 1931, before being taken over by Toyota in 1967.

News of the suspension, which also included several models made under the Toyota name, sent shares in the auto titan tumbling almost four percent in Tokyo on Thursday.

The losses were also spurred by news that the world's biggest carmaker was recalling around a million Toyota and Lexus vehicles in the United States, citing concerns about their airbag systems.