The Guardian newspaper says the British eavesdropping agency GCHQ repeatedly hacked into foreign diplomats' phones and emails when the U.K. hosted international conferences, even going so far as to set up a bugged Internet café in an effort to get an edge in high-stakes negotiations.

The report — the latest in a series of revelations which have ignited a worldwide debate over the scope of Western intelligence gathering — came just hours before Britain was due to open the G8 summit Monday, a meeting of the seven biggest economies plus Russia, in Northern Ireland. The allegation that the United Kingdom has previously used its position as host to spy on its allies and other attendees could make for awkward conversation as the delegates arrive for talks.

"The diplomatic fallout from this could be considerable," said British academic Richard J. Aldrich, whose book GCHQ charts the agency's history.

The Guardian cites more than half a dozen internal government documents provided by former NSA contractor Edward Snowden as the basis for its report on GCHQ hacking activities. (Ewen MacAskill/Associated Press, The Guardian)

GCHQ declined to comment on the report.

The Guardian cites more than half a dozen internal government documents provided by former NSA contractor Edward Snowden as the basis for its reporting on GCHQ's intelligence operations, which it says involved, among other things, hacking into the South African foreign ministry's computer network and targeting the Turkish delegation at the 2009 G20 summit in London.

The source material — whose authenticity could not immediately be determined — appears to be a mixed bag. The Guardian describes one as "a PowerPoint slide," another as "a briefing paper" and others simply as "documents."

Some of the leaked material was posted to the Guardian's website with heavy redactions. A spokesman for the newspaper said that the redactions were made at the newspaper's initiative, but declined to elaborate.

Hacking tactics 'a bit Mission Impossible'

It wasn't completely clear how Snowden would have had access to the British intelligence documents, although in one article the Guardian mentions that source material was drawn from a top-secret internal network shared by GCHQ and the NSA. Aldrich said he wouldn't be surprised if the GCHQ material came from a shared network accessed by Snowden, explaining that the NSA and GCHQ collaborated so closely that in some areas the two agencies effectively operated as one.

One document cited by the Guardian — but not posted to its website — appeared to boast of GCHQ's tapping into smartphones. The Guardian quoted the document as saying that "capabilities against BlackBerry provided advance copies of G20 briefings to ministers." It went on to say that "Diplomatic targets from all nations have an MO (a habit) of using smartphones," adding that spies "exploited this use at the G20 meetings last year."

Another document cited — but also not posted — concerned GCHQ's use of a customized Internet café which was "able to extract key logging info, providing creds for delegates, meaning we have sustained intelligence options against them even after conference has finished." No further details were given, but the reference to key logging suggested that computers at the café would have been pre-installed with malicious software designed to spy on key strokes, steal passwords, and eavesdrop on emails.

Aldrich said that revelation stuck out as particularly ingenious.

"It's a bit Mission Impossible," he said.