Toyota Motor reportedly has said it would suspend shipments of some models including the Hilux truck and Land Cruiser 300 SUV after finding irregularities in certification tests for diesel engines developed by affiliate Toyota Industries.

A special investigative committee found irregularities during horsepower output testing for the certification of three diesel engine models, Reuters reported.

Ten models use the affected engines worldwide including the Hiace van, Fortuner SUV, Innova multi-purpose vehicle and Lexus LX500D SUV.

According to the report, Toyota said it sought to thoroughly explain the matter to authorities and would take measures such as running new engine certification tests in the presence of regulators as needed.

"There was a lack of communication with Toyota Motor and not enough coordination about testing processes and procedures that should have been followed," Toyota Industries president Koichi Ito reportedly said at a press conference.

Toyota Industries said it sold about 84,000 affected automobile diesel engines during the financial year to 31 March, 2023.

Toyota CEO Koji Sato told Reuters the company used 36,000 of the affected engine models worldwide each month, which equals 432,000 units a year.

A Toyota spokesperson could not immediately tell the news agency how many affected vehicles the automaker had sold over the years.

In Japan, the shipment halt impacted production at six lines at four plants, Reuters reported.

The company said it had re-confirmed that affected engines and vehicles met engine performance output standards.

It added the investigation had found that electronic control units used during horsepower output testing were different from those used during engine production.

Japan's transport ministry told Reuters it would on Tuesday conduct an on-site investigation of Toyota Industries' Hekinan plant in central Aichi prefecture where the company makes automotive and industrial engines.

Toyota's investigation initially focused on certification regulations relating to emissions performance of forklift and construction machinery engines, in which the special investigative committee also confirmed misdoing, Reuters said.

The automaker holds a near 25% stake in Toyota Industries, which is a key Toyota group company. The supplier in turn owns about 8% of Toyota shares itself, according to the report.