The British spy agency GCHQ has secretly tapped more than 200 fiber-optic cables carrying phone and internet traffic and has been sharing data with the U.S. National Security Agency, according to a news report.

The spy operation, which included placing intercepts at the landing points of transatlantic undersea cables where they surface in the U.K., has allowed the Government Communications Headquarters (GCHQ) to become the top interceptor of phone and internet data in the world, according to the Guardian newspaper, which broke the story based on documents leaked by former NSA systems administrator Edward Snowden.

One part of the operation, codenamed Tempora, has been operating for about 18 months and allows the agency to tap large volumes of data siphoned from the cables and store it for up to 30 days for sifting and analyzing.

Each of the cables carries about 10 gigabits of data per second, which the paper likened to sending all of the information in all the books in the British Library through the cables 192 times every 24 hours.

Filters allow the agency to reduce the amount of traffic it records – one filter cuts out about 30 percent of traffic just by eliminating peer-to-peer downloads – while still collecting vast amounts of data that get sifted by analysts.

Some 850,000 NSA employees and U.S. private contractors with top secret clearance have access to GCHQ databases and as of May last year, at least 750 analysts from the U.K. and NSA were tasked specifically with sifting through the data, using more than 70,000 search terms related to security, terrorist activity and organized crime. Search terms focus on subjects, phone numbers and email addresses of interest.

The tapping was conducted in cooperation with commercial companies that own and operate the cables, the paper noted.

"There's an overarching condition of the licensing of the companies that they have to co-operate in this," an unnamed source told the paper. "Should they decline, we can compel them to do so. They have no choice."

The tapping began as a trial in 2008 and within two years the GCHQ achieved top eavesdropper status among the nations known as the Five Eyes of electronic eavesdropping – U.S., U.K., Canada, Australia and New Zealand. GCHQ reportedly now "produces larger amounts of metadata than NSA" as a result of the program.

During a 2008 visit to the GCHQ's listening station at Menwith Hill NSA Director Gen. Keith Alexander reportedly remarked: "Why can't we collect all the signals all the time? Sounds like a good summer project for Menwith."

The program has been justified for allowing the agencies to identify new techniques used by terrorists to thwart security checks, to uncover terrorist activities during the planning stages and to track child exploitation networks and aid in cybersecurity defenses against network attacks.