After months of denial and an inexplicably huge surge in home runs, Japan's baseball chiefs have admitted they secretly switched the design of the ball to make the game more exciting.

Players and fans had repeatedly quizzed Nippon Professional Baseball (NPB) bosses after seeing a 40 percent rise in the number of balls that were slugged out of the park so far this season.

In April, NPB said the specifications of their ball, each of which bears the signature of its commissioner Ryozo Kato, "have not been changed," a statement that was repeated several times since.

But on Tuesday NPB came clean, saying they had asked manufacturer Mizu?no 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."

Commissioner Kato, a former Japanese ambassador to Washington, 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.

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

At the time, the organization 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. "They pointed out that the game had become uninteresting. It has been a matter under discussion," Shimoda said.

The Japan Professional Baseball Players Association wanted to know why an average of 1.50 runs were being scored in 2013, against 1.05 in the previous two years.

Mizuno initially said the increase was due to foreign batters hitting so many home runs and was also related to the higher number of games being played in domed stadiums, where wind is not a factor, Kyodo News reported.

But union chairman Motohiro Shima said it was important the organization was honest because it affected statistics.

Japanese media on Wednesday lashed out at the NPB commissioner, accusing him of lying to the baseball-crazy nation.

"The commissioner's words and deeds, which lacked probity, raise questions over whether he should resign," the influential daily Asahi Shimbun said.

Baseball is the most widely-followed professional sport in Japan, with games shown regularly on television.