TOKYO, Aug. 9 (Xinhua) -- Three major Japanese makers apologized Thursday for conducting improper inspections of new vehicles, dealing another blow to the reputation of the troubled Japanese manufacturing industry.

Suzuki Motor Corp., Mazda Motor Corp. and Yamaha Motor Co. admitted at separate press conferences on Thursday that they have cleared vehicles for emissions or fuel efficiency standards after improper tests.

Suzuki President Toshihiro Suzuki told a press conference in Tokyo that invalid fuel-economy or emission test data had been taken as valid on about half of the 12,819 vehicles sampled for tests.

Yamaha Motor Executive Vice President Katsuaki Watanabe admitted at a separate press conference that data on fuel efficiency of seven motorcycles were invalid and required retests.

Mazda also confirmed on the same day that it had conducted problematic fuel-economy inspection for 72 cars.

All the three vehicle makers denied deliberate data alterations but attributed the problems to human errors or lack of training of the workers.

They also said that they will not recall any of the vehicles as fuel economy or emission standards of the vehicles were not affected by the improper tests.

The misconduct came into light after Japan's transport ministry last month instructed 23 vehicle makers to conduct internal investigations after Nissan Motor Co. and Subaru Corp. admitted to falsifying emissions or fuel efficiency data.

"It is extremely regrettable," Keiichi Ishii, Minister of Land, Infrastructure, Transport and Tourism said in a statement, adding that the ministry will demand the manufacturers to take measures to prevent such misconduct from happening again.

Shares of the three companies tumbled on the Tokyo Stock Exchange on Thursday, with Suzuki down 6.0 percent, Yamaha falling 4.6 percent and Mazda shedding 1.3 percent.

The incident followed a series of scandals involving major Japanese makers, including data falsification at Mitsubishi Materials Corp. and Kobe Steel Ltd., and uncertified safety checks carried out by Nissan Motor Co. and Subaru Corp., which came into light last year and tarnished the reputation of Japanese manufacturing industry.