Japan's central bank is investigating a high-profile media report that revealed it was mulling a negative interest rate policy just before it made the formal announcement. 

The country's leading Nikkei business daily published its story online on Friday, several minutes before the central bank shocked markets with the surprise stimulus after a closed-door meeting. 

The potential leak sent the yen into a steep dive and sparked a huge rally in Tokyo share markets as investors cheered the Bank of Japan's newest weapon in its bid to kickstart the world's number three economy. 

"We have started investigating the matter," said Bank of Japan spokesman Tadaaki Kumagai. 

"We are interviewing BoJ officials who were in a position to know policy discussions and some government officials as they were also participating the meeting," he added. 

The Bank of Japan's negative rate decision - effectively charging commercial lenders to park new deposits at the bank - is aimed at ramping up lending to people and businesses in order to kickstart the economy. 

It was adopted by a narrow 5-4 vote, a result hinted at in the Nikkei report which said that Bank of Japan chief Haruhiko Kuroda was trying to convince some of his sceptical colleagues. 

The bank's meetings are considered top secret with media waiting in a closed room with no outside communication until the board releases its policy decision.

The influential paper's report "came before we handed out printed statements to journalists, so it's not a case of breaching an embargo", the Bank of Japan spokesman said. 

The Nikkei, which bought the Financial Times last year, is known for its accuracy in reporting corporate earnings results weeks before official announcements by companies. 

Foreign investors have criticised the early reports, saying they give their Japanese counterparts an unfair home advantage being many time zones ahead, and are usually published first in Japanese. 

In December, the Financial Times caused a stir on financial markets after erroneously reporting that the European Central Bank was to leave interest rates unchanged, minutes before the ECB actually cut a key rate.