Japan's Finance Ministry has uncovered evidence of a major Trojan cyber-attack on its computer systems that lay undetected for almost two years, according to local sources.

Ministry officials have admitted that the unspecified Trojan, which was not detected by the organisation's security systems, was probably free to steal confidential data from January 2010 to November 2011,  after which the attack suddenly stopped.

A total of 123 computers inside the Ministry were infected out of around 2,000 so far checked, sources stated, which prompted the organisation to change hard disks on the affected machines.

Officials have not speculated on who might have carried out the attack, which seems to have affected only relatively junior staff, but the unspoken candidate will be China.

The latest attack bears the hallmark of a large number that have afflicted the Japanese Government departments in the last year, seemingly only discovered after the damage was done.

These included one of several raids on Japanese defence contractors in 2011, the country's Parliament, indeed almost any wing of the industrial base that might be of interest to foreign powers.

Possibly in response to such targeting, earlier this year, the Japanese admitted they were testing a new cyber-defence weapon.