Japan's Mazda, Suzuki and Yamaha have admitted to conducting improper fuel economy and emissions tests on their vehicles in Japan, in revelations which pile further misery on Japan's beleaguered manufacturing sector.

The companies came forward after the transport ministry last month ordered 23 auto and motorbike companies to conduct in-house probes after it emerged Nissan and Subaru had cheated on fuel economy and emissions data.

All three reported "inappropriate handling" of vehicle inspections, the ministry said.

They said incomplete emissions tests were done on some of its vehicles, but its officials certified the results as though the tests had been administered properly.

Suzuki admitted improper inspections on 6,401 vehicles, or nearly half of those subject to sample checking, between 2012 and 2018.