Japanese baseball officials have admitted switching match balls to give greater bounce and create more excitement during games.

Players and fans have repeatedly quizzed Nippon Professional Baseball (NPB) bosses after seeing a 40 per cent rise in the number of balls hit out of the park so far this season.

In April the NPB said the specifications of their ball "have not been changed" in a statement that was repeated several times since.

Wayne Graczyk, baseball columnist for the Japan Times, has told Connect Asia the revelations are "not a big surprise".

"Two weeks into the season it became obvious that something had happened," he said.

"We thought maybe they put the old ball back and just didn't say anything, and apparently that's exactly what had happened."

On Tuesday the NPB said it had asked manufacturer Mizuno to "adjust" the ball to give it greater bounce off the bat and demanded the company keep quiet about the switch.

"Our understanding was that it would be a matter of fine-tuning," NPB secretary general Kunio Shimoda said.

"We thought it would cause confusion if we let it be known."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 42 seconds 3 m 42 s Wayne Graczyk speaks to Connect Asia ( Connect Asia ) Download 1.7 MB

Mr Kato originally ordered a change in the make-up of the ball back in 2011, to bring it into line with the model used in the US.

The cork core of the ball was wrapped with a low-resilience rubber and its seams were widened.

NPB also made Mizuno the sole ball supplier, dropping its three rivals.

At the time, the organisation said the change would help Japanese players get accustomed to international standards.

However, the switch resulted in a sharp fall in home runs - 939 in 2011 and 881 in 2012 compared with 1,605 in 2010.

Last year the players' union asked clubs to review the sluggish ball.

Mr Graczyk says NPB's mistake could cost commissioner Ryozo Kato - whose signature appears on every ball - his job, despite his efforts to keep the story "low key".

"The mistake was not changing the ball back, the mistake was not announcing it," he said.

"I would say there's a chance he may have to resign over this because it's such a huge story and embarrassment for Japanese baseball."

Mr Kato on Wednesday denied he knew anything about the latest ball change, although he apologised for the incident.

"I didn't know anything about it at all," he said.

"I have never intended to cover up the fact at all. Please allow me to apologise for causing such confusion as this."

ABC/AFP