Britain, working with the United States' National Security Agency, intercepted phone calls and monitored computers used by officials taking part in two high-level international finance meetings in London in 2009, a British newspaper said on Sunday.
The latest report, leaked by former NSA contractor Edward Snowden, reveals that the U.S. specifically targeted Russian President Dmitry Medvedev's communication signals, or "meta-data," back to the Russian embassy in London.
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The effort, however, apparently didn't apparently reveal much beyond "a change in the way Russian leadership signals have been normally transmitted," the Guardian reported.
According to the newspaper, the NSA documents stated that "this is an analysis of signal activity in support of President Dmitry Medvedev's visit to London. The report details a change in the way Russian leadership signals have been normally transmitted. The signal activity was found to be emanating from the Russian embassy in London and the communications are believed to be in support of the Russian president."
Specifically, the details of the intercepts were in a briefing document prepared by the NSA and shared with high-ranking officials from Britain, Australia, Canada and New Zealand.
The disclosure reveals the importance of "the U.S. spy hub at RAF Menwith Hill in Harrogate, North Yorkshire, where hundreds of NSA analysts are based," working alongside liaison officers from British intelligence, the paper concluded.
The Guardian said some delegates from countries in the Group of 20 - which comprises top economies around the world - used Internet cafes that had been set up by British intelligence agencies to read their emails.
The report Sunday was published hours before leaders of the Group of Eight countries - all of which are in the G20 - start a two-day summit in Northern Ireland.
The Guardian said it had seen classified documents that detailed secret monitoring by British intelligence of officials at a G20 leaders summit and a finance ministers' meeting in 2009 and suggested it had been sanctioned at a senior level by the government of former prime minister Gordon Brown.
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The aim of the monitoring, which included tracing who was calling who, appeared to have been to get an edge in the meetings and targets included South Africa and Turkey, the report said.
A spokesman for Britain's foreign ministry declined to comment. The Labour party, which held power in 2009, was not immediately available for comment.
The Guardian this month reported details of surveillance by the NSA of phone records and Internet data in the United States.
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