Bankrupt Canadian telecom company Nortel was penetrated for at least a decade by hackers believed to have been operating from China, media reported.
The hackers, using seven passwords stolen from top executives, including the company's CEO, downloaded technical papers, research and development reports, employee emails and other documents, media said.
It cited Brian Shields, a former 19-year veteran of the firm who led an internal investigation, as saying: "They had access to everything... They had plenty of time. All they had to do was figure out what they wanted."
Shields said the hackers also hid spying software so deeply on employee computers that it took the company years to figure out the extent of the problem.
The revelations highlight the threat posed by Chinese hackers, whom US intelligence said were the world's "most active and persistent perpetrators" of economic espionage in a report submitted to the US Congress last November.
Media quoted the Chinese embassy as denying allegations of cyberspying, saying such attacks are "transnational and anonymous."
The story came out hours before Chinese Vice President Xi Jinping -- expected to become China's next president in 2013 -- was to meet US President Barack Obama at the start of a week-long US tour.