Operations at four Toyota Motor plants in Japan will remain suspended until Monday due to rigged engine certification tests at a group firm.

Toyota halted six production lines after irregularities at Toyota Industries came to light earlier this week.

The testing irregularities involve three types of diesel engines made by Toyota Industries. The automaker has stopped shipping 10 models that use the engines including six for the Japanese market.

Toyota says it will decide on Monday afternoon whether to resume operations at the plants.