Apple and Amazon are among US companies and agencies who have had data stolen by Chinese spies, claims Bloomberg.
The data had been siphoned off via tiny chips inserted on server circuit boards made by a company called Super Micro Computer, reported the news agency.
The servers had been compromised during manufacturing and the chips activated once they were up and running, it said.
Apple, Amazon and Super Micro have rejected Bloomberg's claims, calling them "untrue".
In particular, Apple released a strong statement in response to Bloomberg's article saying it had found "no evidence" to support the allegations.
Bloomberg said a year-long investigation by reporters Jordan Robertson and Michael Riley had uncovered evidence of the wide-ranging attack, which gave Beijing access to 30 large companies and many federal agencies.