Toyota Motor says it's suspending production at four domestic plants after finding what is called "irregularities" in engine certification tests at a group company. Officials at the leading Japanese automaker say the company will halt output at six production lines in total until Thursday.

Sato Koji, President of Toyota Motor, spoke to reporters on Monday. "We will make earnest efforts to restart production as soon as possible by ensuring the right procedures are followed to obtain certification," he said.

The testing irregularities involve three types of diesel engines made by Toyota Industries.

The certification doubts had already prompted Toyota on Monday to suspend shipments of 10 models, including six for the Japanese market.