The bank sent out an e-mail to its key global investors to tell them about the planned merger with Dai Ichi Kangyo and Industrial Bank of Japan.

But the e-mails contained a virus that meant any of the big money men and women who opened it got a surprise message "from Fuji Bank" telling them they were "a big, stupid jerk".

Some of the names in the original message had also changed to "Dr Mountain Dew".

Security 'under control'

"We of course apologised by sending out e-mails immediately after it happened," said a Fuji Bank spokesman.

The bank insisted the virus was not a serious breach of its security.

The spokesman said: "Our system is under perfect control.

"As the virus was sent from a personal computer which was temporarily used outside the main terminal, it was not related to our main network system in any way."

A bank official told the BBC that its computer system had spotted the virus quickly.

The company denied that "Dr Mountain Dew" might be an employee aggrieved at the merger plans.

It said the virus had come from an outside source, the contract to publicise the deal having been "outsourced" to experts.