There is nothing to indicate that the journalists were intentionally targeted, the newspaper reported.
They e-mails were saved by GCHQ -- the British equivalent of the National Security Agency -- and shared on the agency's intranet as part of a test exercise, the Guardian reported.
GCHQ apparently obtained the data from one of its many taps on the fiber-optic cables that comprise the backbone of the Internet.
The disclosure comes as the United States and Britain face pressure to limit government snooping into the confidential communications of reporters. In Washington, Attorney General Eric H. Holder Jr. recently announced tighter guidelines for the use of subpoenas, court orders and search warrants to obtain information and records of journalists.
In Britain, senior editors and lawyers have called for the introduction of a freedom of expression law in response to growing concerns about police abuse of surveillance powers linked to Britain’s Regulation of Investigatory Powers Act.
The communications gathered by GCHQ were sometimes innocuous mass e-mails sent to dozens of journalists by public relations firms, but also included correspondence between reporters and editors discussing stories.
The e-mail collection apparently was part of the testing of a tool to strip irrelevant data out of the agency’s automated collection process, the newspaper reported.
More than 100 editors in Britain have signed a letter to Prime Minister David Cameron protesting recent cases of warrantless surveillance of journalists’ communications.
In the United States, news media and press freedom advocates were outraged two years ago over disclosures that Justice Department officials obtained records from more than 20 phone lines assigned to the Associated Press and its journalists as part of a leak investigation of a failed al-Qaeda plot. In a second leak investigation, a Fox News reporter was called a possible “co-conspirator” in a crime to obtain a search warrant for his records.
Under the new guidelines, federal investigators will need authorization from the attorney general if they want to seek information from journalists who used classified material or confidential sources. Previously, the department had insisted that that policy apply only to “ordinary newsgathering.”
The revelation comes in the wake of terrorist attacks in Paris on a satirical magazine, Charlie Hebdo, and on other targets that claimed 17 lives. Authorities in France and Britain have called for greater surveillance powers.
New evidence from other British intelligence documents obtained by Snowden, who worked as a contractor for the NSA, shows that a GCHQ information security assessment listed “investigative journalists” as a threat alongside terrorists or hackers, the Guardian reported.
Civil liberties advocates reacted sharply to the news. "This is the inevitable result of the kind of wholesale surveillance that's now authorized under both U.K. and U.S. law," said Jameel Jaffer, deputy legal director of the American Civil Liberties Union.
"Both the GCHQ and NSA collect huge volumes of communications between people who have no connection at all to terrorism, and those communications include the e-mails and phone calls of attorneys, human rights researchers and journalists," Jaffer said. "The back-end restrictions that are meant to mitigate the privacy intrusion are weak by design and riddled with exceptions. There is little evidence that this kind of mass surveillance is effective or necessary, but it presents a very real threat not just to privacy but to the freedoms of speech and association as well."
Press freedom activists said the disclosure raises questions. "At first glance, it doesn't look like reporters were intentionally targeted, but we are concerned about whether under information-sharing agreements the United States has with Britain, a U.S. intelligence agency could have gotten ahold of this information," said Hannah Bloch-Wehba, Stanton Foundation fellow at the Reporters Committee for Freedom of the Press.