"Emails from the BBC, Reuters, the Guardian, the New York Times, Le Monde, the Sun, NBC and the Washington Post were saved by GCHQ and shared on the agency's intranet as part of a test exercise by the signals intelligence agency," the Guardian reported.
The emails were among 70,000 gathered in the space of less than 10 minutes on one day in November 2008 by the spy agency, the paper said, citing documents leaked by US National Security Agency's former contractor Edward Snowden.
The communications were gathered in one of many taps of the fibre-optic cables that form the Internet's backbone, and were available for viewing by any cleared staff on (The Government Communications Headquarters) GCHQ's intranet, according to the report.
The emails appeared to have been captured and stored as the output of a then-new tool being used to strip irrelevant data out of the agency's tapping process.
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The paper said there is nothing to indicate whether or not the journalists were intentionally targeted.
"All of GCHQ's work is carried out in accordance with a strict legal and policy framework, which ensures that our activities are authorised, necessary and proportionate, and that there is rigorous oversight, including from the secretary of state," the paper quoted a spokesman for GCHQ as saying.
Senior editors and lawyers in the UK have called for the urgent introduction of a freedom of expression law amid growing concern over safeguards proposed by ministers to meet concerns over the police use of surveillance powers linked to the Regulation of Investigatory Powers Act 2000 (Ripa).