Britain's electronic spy agency GCHQ tapped emails of journalists at some of the world's top media organisations, the Guardian reported on Monday.
The report said GCHQ gathered emails from journalists at the BBC, the Guardian, Le Monde, NBC, the New York Times, the Sun, Reuters and the Washington Post.
The emails were among 70,000 gathered in less than 10 minutes in 2008 by the spy agency, Britain's equivalent of the US National Security Agency, according to the Guardian's analysis of documents leaked by former US intelligence contractor Edward Snowden.
The emails 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 GCHQ's intranet, according to the report.
The spy agency considers journalists as "a potential threat to security" according to internal security advice cited by the Guardian, with investigative journalists listed as a threat alongside terrorists and hackers.
The report was published amid pressure to limit the British government's ability to spy on journalists' communications, after revelations police accessed phone records to identify journalists' sources within the police.
Over 100 editors of British newspapers published a joint letter on Monday calling on the government to stop law enforcement officials viewing journalists' phone records without a warrant from a judge.
Prime Minister David Cameron renewed calls for increased powers of surveillance in the wake of attacks by extremists in Paris that left 17 dead, saying terrorists should be denied a "safe space" to communicate.