In July, the company denied a German television report that it had cooperated with American intelligence agencies to spy on German citizens using its network. The New York Times reported in September that for at least three years, GCHQ had been working to gain access to traffic in and out of data centers operated by Google, Yahoo, Facebook and Microsoft’s Hotmail. The program, described as having been developed in close collaboration with the N.S.A., was said to have achieved “new access opportunities” into Google’s systems by 2012, according to GCHQ documents provided by Mr. Snowden. But it was not clear what that meant.

The Post said that under a system code-named Muscular, GCHQ was storing data taken in from the interception in a rolling three- to five-day “buffer,” during which the two agencies decoded it and filtered out information they wanted to keep.

It also reported that the N.S.A. was using about 100,000 “selectors” as its search term filters — more than twice as many, it said, as the agency has been using from its Prism program inside the United States. In that program, the agency collects emails, search queries and other online activity of foreigners abroad from Google, Yahoo and other companies through a court-approved process authorized by the FISA Amendments Act of 2008.

GCHQ documents obtained from Mr. Snowden by The Guardian newspaper and shared with The Times reveal an intense focus over several years by British spies on the development of Muscular and a closely related project code-named Incenser. The documents suggest that both programs are to a large extent driven by N.S.A. intelligence needs and are highly prized by the Americans.

In November 2010, the British wrote that “Muscular/Incenser has significantly enhanced the amount of benefit that the N.S.A. derive from our special source accesses.” Those projects in some cases provide data that are unavailable from any other source, one document said, “highlighting the unique contribution we are now making to N.S.A., providing insights into some of their highest priority targets.”

In its article, The Post described a January document as saying that the N.S.A.’s headquarters in Fort Meade, Md., was taking in more than 180 million records a month from the project. It also reported that briefing documents said collection from Yahoo and Google had produced important intelligence leads against hostile foreign governments.