Britain's spy agency GCHQ has tapped fibre-optic cables that carry international phone and internet traffic and is sharing vast quantities of personal information with the U.S. National Security Agency, the Guardian newspaper said on Friday.
The paper, which has in recent weeks been publishing details of top-secret surveillance programs exposed by former NSA contractor Edward Snowden, said on its website that Snowden had shown it documents about a project codenamed "Tempora."
Tempora has been running for about 18 months and allows GCHQ, which stands for Government Communications Headquarters, to tap into and store huge volumes of data drawn from fibre-optic cables for up to 30 days, the paper said.
The Guardian said Snowden had provided it with access to documents about GCHQ's alleged cable-tapping operation as part of his effort to expose "the largest program of suspicionless surveillance in human history."
For decades, the NSA and GCHQ have worked as close partners, sharing intelligence under an arrangement known as the UKUSA agreement. They also collaborate with eavesdropping agencies in Canada, Australia and New Zealand under an arrangement known as the "Five Eyes" alliance.
The latest Guardian story will likely put more pressure on British Prime Minister David Cameron's government to reassure the public about how data about them is collected and used.
Earlier this month, in response to questions about the secret U.S. data-monitoring program Prism, British Foreign Secretary William Hague told Parliament that GCHQ always adhered to British law when processing data gained from eavesdropping.
He would not confirm or deny any details of UK-U.S. intelligence sharing, saying that to do so could help Britain's enemies.
"In line with long-standing practice we do not comment on intelligence matters," a GCHQ spokesman said on Friday.
NSA spokeswoman Judith Emmel rejected any suggestion the U.S. agency used the British to do things the NSA cannot do legally. Under U.S. law, the NSA must get authorization from a secret federal court to collect information either in bulk or on specific people.
"Any allegation that NSA relies on its foreign partners to circumvent U.S. law is absolutely false. NSA does not ask its foreign partners to undertake any intelligence activity that the U.S. government would be legally prohibited from undertaking itself," Emmel said.
The Tempora operation involves attaching intercept probes to transatlantic cables where they land on British shores from North America, the Guardian said.
That was done with the agreement of unnamed companies, which were forbidden from revealing warrants that compelled them to allow GCHQ access, it added.
Snowden made world headlines earlier this month when he provided details of NSA surveillance programs to the Guardian and the Washington Post.
In Washington, Snowden's disclosures have ignited a political storm over the balance between privacy rights and national security, but the NSA has defended the programs, saying they have disrupted possible attacks.
In the wake of Snowden's revelations, U.S. officials acknowledged that the NSA, with cooperation from internet and telephone companies, collected email on foreign intelligence suspects, including counterterrorism targets, as well as masses of raw data on calls made within the United States and overseas by subscribers to major telephone companies.
The content of messages of people in the United States - including U.S. citizens - sometimes are intercepted "incidentally," officials have said, but rules require such intercepts to be purged unless U.S. authorities get court authorization.