Computers in Japan's lower house of parliament were hit by cyberattacks from a server based in China that left information exposed for at least a month, a report said today.

Passwords and other information could have been compromised in the attacks, which began in July but were not reported to security authorities until the end of August, the Asahi Shimbun said, without citing sources.

The government's top spokesman Osamu Fujimura, chief cabinet secretary and a lower house member, said he was not previously aware of the reported attack but that the government was investigating the issue.

However, the Asahi said that lower house officials told lawmakers and parliamentary staff to change their network IDs and passwords, over fears that security had been breached.

The Asahi said politicians' computers and a lower house server contracted a "Trojan horse" virus containing a programme that allowed a China-based server to steal passwords and other information.

It was not clear who was behind the attack, it said, adding it was possible the China-based server could have been controlled from a third country.

The cyberattack began when a lower house politician opened an attachment to an e-mail in late July, the newspaper said, adding that the unnamed lawmaker did not report the suspected virus infection until late August.

Fujimura said the government was checking the facts of the reported issue.

"If criminal acts are confirmed, police will strictly deal with it," he told a regular press briefing.

The report came as Japan probes a series of recent cyberattacks on defence contractor Mitsubishi Heavy, which the Asahi on Monday said could have resulted in the theft of information on military aircraft and nuclear power plants.

China has been accused of spearheading online attacks on government agencies and companies, allegations Beijing has always denied.

In June, Internet giant Google said a cyber-spying campaign originating in China had targeted the Gmail accounts of senior US officials, military personnel, journalists and Chinese political activists.