London (CNN) -- Britain's equivalent to the U.S. National Security Agency, the Government Communications Headquarters, has tapped into many of the world's key international fiber optic cables and is routinely downloading and analyzing vast quantities of Internet and phone traffic, sharing the data with the NSA, The Guardian newspaper reported Friday.
The NSA slammed the report as "absolutely false."
"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," NSA spokeswoman Judith Emmel said.
The scope of the surveillance dragnet described in the article is enormous.
The newspaper says the report, like many previous ones, is based on the Guardian's reading of documents provided by former U.S. defense contractor Edward Snowden, who admitted leaking documents this month detailing government surveillance programs.
Unlike some previous reports, the paper has not published the full documents on which the story was based.
A spokesman for the British agency, known as GCHQ, issued a statement saying that in line with long-standing practice, it does not comment on intelligence matters.
"It is worth pointing out that GCHQ takes its obligations under the law very seriously," the statement read. "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, including from the secretary of state, the Interception and Intelligence Services Commissioners and the Intelligence and Security Committee."
The prime minister's office at 10 Downing Street also gave a statement saying only, "We don't comment on intelligence matters."
The GCHQ is one of the three UK intelligence agencies and, according to its website, forms a "crucial part of the UK's national intelligence and security machinery."
A source with knowledge of intelligence matters said "intelligence agencies are there to keep citizens safe and the vast majority of data collected is discarded."
The process used by the GCHQ, the source said, "scans bulk data for any information that can have national security implication.
"Only information deemed useful for national security is pulled out and examined in more detail. The vast majority of data is not examined or retained.
"The process is legal and governed by the 2000 Regulatory Investigatory Power Act. It is regularly reviewed and authorized by ministerial warrants. This is vital national security work. It's proportionate and it's about following terrorist or criminal activity and not about following law-abiding citizens."
CNN's Bharati Naik contributed to this report.