Japan's largest weapons supplier might have breached multi-million dollar supply contracts by not revealing until today that it was the victim of a cyber attack last month.

Mitsubishi Heavy Industries (MHI) has confirmed that at least eight different viruses were found on 45 servers and 38 computers belonging to the weapons supplier. The cyber attack reportedly targeted data on missiles, submarines and nuclear power plants.

It is not believed that any sensitive information was obtained from the attack, but the government has demanded that MHI carry out a full investigation A spokesman quoted by Reuters said: "It's up to the defence ministry to decide whether or not the information is important. That is not for Mitsubishi Heavy to decide. A report should have been made."

A shipyard in Nagasaki where destroyers are built and a facility in Kobe that manufactures parts for nuclear power stations were targeted by the viruses.

As all government contractors are obliged to disclose any breach such as this it seems that MHI did not think the attack was severe enough to warrant a report, or it did not want the negative publicity that would be associated with disclosing such an attack.

The attacks, which are believed to have originated outside of MHI's computer network, have been described as spear phishing attacks, where the hackers send personalised emails to victims and link them to a fake website, where they are encouraged to enter their financial details or other sensitive information.

No one has yet claimed responsible for the hacking, but one Japanese newspaper said Chinese language script was found in the attacks. China has quickly reacted to this accusation: "Criticising China as being the source of hacking attacks not only is baseless, it is also not beneficial for promoting international co-operation for internet security"

The BBC has reported that Japanese government websites have been hit by a distributed denial-of-service attack (DDoS). These attacks use thousands of hacker-controlled computers to hit a website simultaneously, causing it to collapse under the extreme load.

MHI secured over 200 government contracts last year and one tenth of its income comes from the domestic weapons sector.

The hackers also targeted IHI, a contractor which supplies engine parts for military aircraft, but were stopped by its security systems.