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 Snowden had shown it documents about a project codenamed "Tempora."

Tempora has been running for around 18 months and allows GCHQ to tap into and store huge volumes of data drawn from fibre-optic cables for up to 30 days, the paper said.

The story is likely to put further pressure on 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, Foreign Secretary William Hague told parliament that GCHQ always adhered to British law when processing data gained from eavesdropping.

Britain's Government Communications Headquarters in Cheltenham, England, is seen in this undated handout aerial photograph released in London on October 18, 2010. (Crown Copyright/Reuters)

He would not confirm or deny any details of U.K.-U.S. intelligence sharing, saying that to do so could help Britain's enemies.

"In line with longstanding practice we do not comment on intelligence matters," a GCHQ spokesman said on Friday.

"It is worth pointing out that GCHQ takes its obligations under the law very seriously. Our work is carried out in accordance with a strict legal and policy framework which ensures that our activities are authorized, necessary and proportionate, and that there is rigorous oversight."

Involvement of unnamed companies

The Tempora operation involves attaching intercept probes to transatlantic cables where they land on British shores from North America, the Guardian said.

This was done with the agreement of unnamed companies, who 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.