At a minimum, former CIA Director Michael Hayden said, Huawei provided Chinese officials with "intimate and extensive knowledge of the foreign telecommunications systems," in an interview with Australian Financial Review published Thursday.
"I think that goes without saying -- it's one reality," Hayden told the Australian Financial Review.
Hayden's accusations are the latest in a series of warnings from U.S. officials and congressional committees about the dangers of allowing the Chinese telecom firm to access or build Western networks.
However, the accusations voiced by Hayden, also a former National Security Agency director, are the first to flat-out charge Huawei was spying on Beijing's behalf.
James Lewis, a director of the Center for Strategic and International Studies in Washington, told the Australian Financial Review Hayden's remarks reflected the view of the U.S. government.
"Officials within several agencies have privately told me that Huawei is a national security threat," Lewis said. "While you hear rumors of the evidence, it has never come out in public before."
Hayden told the Australian newspaper that, given the "over-arching national security risks a foreign company building your national telecoms networks creates, the burden of proof is on Huawei," which he said has fallen "well short."
"These guys are not even transparent to themselves," he said. "There's no transparency around who appoints the board or who controls the ownership of the business."
"And there's no independent Chinese government oversight committee that could give us confidence that Huawei would not do what they promised not to do," he told the Financial Review.
Hayden did not provide details to support his claims, but said intelligence agencies have accumulated evidence, CNNMoney said.
Hayden now serves on the board at Motorola Solutions and is a principal at Chertoff Group, a security consultant.
Huawei has denied the charges, saying its products and infrastructure networks are safe and that it doesn't have ties with the Chinese government.
"These tired, unsubstantiated defamatory remarks are sad distractions from real-world concerns related to espionage -- industrial and otherwise -- that demand serious discussion globally," Huawei spokesman Scott Sykes said Friday.
Huawei, among the world's largest telecommunications companies, offers products such as routers and other Internet devices.
Last year, the U.S. House Intelligence Committee issued a report critical of the firm's record of intellectual property violations, its alleged ties to Iran, and what the committee called "a pattern and practice of potentially illegal behavior."
On Thursday, British officials said they would review a security evaluation center established by Huawei in Britain, CNNMoney said. The center was designed to allow for inspection of Huawei products used in Britain's telecommunication networks.
Huawei said it welcomes the review.
"Our work with Huawei and their U.K. customers gives us confidence that the networks in the U.K. that use Huawei equipment are operated to a high standard of security and integrity," Sykes said.