TOKYO (Reuters) - One of Japan's biggest banks said Friday it accidentally e-mailed a computer virus to some global investors last week, but said no harm was done to the investors' or its own computer network systems.

A spokesman for Fuji Bank Ltd said: "It was sent from a particular personal computer which we do not normally use. It's outside our main computer system in which we have built a solid monitoring structure."

The Financial Times said in a front-page report Friday that the virus causes the recipient's computer to display an insulting message on the 14th day of each month calling the viewer a "big stupid jerk."

It said the e-mail message was about Fuji's plan to tie up with two other big Japanese banks, announced on Aug 20.

The Fuji spokesman said the virus had come from outside sources, not from a bank employee.

Fuji, Dai-Ichi Kangyo Bank and Industrial Bank of Japan plan to start a joint holding company next year to create what will be the world's biggest banking group in terms of assets.