The Hitachi Ltd unit said the items included material used in lithium-ion batteries and affected about 1,900 companies.

That was in addition to the roughly 500 companies affected by the initial finding of improper tests on lead-acid batteries in June, the firm said in a statement.

No defects, safety issues or illegal conduct have been found so far, it added.

At a news conference, Chief Executive Hisashi Maruyama bowed deeply and apologised, blaming a lax company culture and saying every effort would be taken to prevent a recurrence.

"I wish to deeply apologise for causing trouble for many people," Maruyama was quoted as saying by Kyodo news agency.

This week, Hitachi Chemical said it had failed to properly test parts used to encapsulate semiconductors, and a panel of external experts was investigating the issue.

Compliance issues first surfaced in June, when Hitachi Chemical found data falsification in quality tests of lead-acid batteries for industrial use over a period of more than seven years, affecting about 60,000 products.

It joins a growing list of Japanese manufacturers including Kobe Steel Ltd, Toray Industries Inc and more recently, KYB Corp, that have admitted to data falsification or other types of misconduct.

The company said the impact on its earnings for the business year to the end of March 2019 was as yet unknown.

(Reporting by Chang-Ran Kim and Junko Fujita; Editing by Christian Schmollinger and Darren Schuettler)