The cyber attack on Canada comes a year after Canadian investigators reported a similar China-based offensive on India, dozen of other countries and the Dalai Lama to steal secret documents.

At least two key Canadian government departments have been taken off the internet to ward off hackers after the cyber-attack which was detected last month, said the Canadian Broadcasting Corporation (CBC) Wednesday.

According to the TV network, the hackers managed to take control of computers in the offices of senior government executives to steal passwords that unlock entire government data systems.

They successfully penetrated the computer systems at the Finance Department and the Treasury Board. But it is not known whether the hackers managed to access other departmental computer networks containing sensitive personal information such as tax and health records, the network said.

However, as soon as the cyber breach was detected, officials immediately cut off internet access in both departments to prevent any stolen information from being sent back to the hackers over the net. "The attack, first detected in early January, left Canadian counter-espionage agents scrambling to determine how much sensitive government information may have been stolen and by whom," the broadcaster said.

Because of the cyber attack, thousands of public servants in the two departments went without internet access, "although officials in both affected departments report service has slowly been returning to normal since the attack", the report said.

The cyber-attack has been traced to servers in China, the report said. But there is no way of knowing whether it originated in China or was mounted through servers in China, according to the report.   The Canadian government initially shrugged off  the cyber-attack as an "attempt to access" federal networks and have not publicly acknowledged it, the network said.

Last year, Canadian cyber investigators had unearthed an unprecedented Chinese cyber offensive against India, dozens of other nations and the Dalai Lama under which  hackers of the so-called 'Shadow Network' stole secrets files of India's missile projects, troop deployments and military schools.

In what was then termed the world's biggest cyber spy ring, the Canadian investigators in their report "Shadows in the Cloud: An investigation into cyber espionage 2.0" exposed that Chinese hackers used simple methods of e-mail and Twitter to steal secrets from computers around the world. The report was released in April after a year-long investigation.