Documents leaked by US whistleblower Edward Snowden reveal that British intelligence used fake internet cafes to spy on foreign diplomats during an international summit in London.
The papers, seen by the Guardian, also suggest British spies tapped the phones of foreign diplomats and monitored their computers during two separate G20 meetings in the British capital, in April and September 2009.
The surveillance was carried out on the instruction of the British government, which employed dozens of analysts to monitor delegates' phone conversations day and night.
According to two of the documents leaked by Snowden, who is currently in hiding Hong Kong, the intelligence garnered from the conference was directly passed to "ministers" by GCHQ staff.
The leak comes at an embarrassing time for Britain, which is currently hosting another international summit - the meeting of the G8 in Belfast.
The documents suggests that the internet cafes set up by GCHQ, MI6 and other agencies used an e-mail interception programme, as well as software to log the keystrokes made by delegates as they sent messages.
One intelligence staffer wrote that, through the cafe, "[we] were able to extract key logging info, providing creds for delegates, meaning we have sustained intelligence options against them even after conference has finished". This suggests GCHQ retained delegates' log-in details for possible future surveillance.
Other documents suggest that smartphones were also targeted. One note read: "New converged events capabilities against BlackBerry provided advance copies of G20 briefings to ministers ... Diplomatic targets from all nations have an MO of using smartphones. Exploited this use at the G20 meetings last year."
By the time the second meeting began in September, it appears surveillance experts had developed new technology to provide a live report on all delegate telephone conversations.
According to the Guardian, the minute-by-minute report was beamed onto a 15 sq m video wall at GCHQ's operations centre, and onto the screens of 45 specialist analysts.
Meanwhile, an NSA briefing note suggests Dmitry Medvedev, the then-president of Russia, was the the principal target for surveillance officials.
The document says that NSA staff intercepted communications from Medvedev's delegation the day he arrived in London - hours after he had met US President Barack Obama for the first time.
The author continued: "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."
Russia's current president, Vladimir Putin, is attending today's G8 conference in Belfast, and has already stirred rancour by comparing the opponents of Syrian President Bashar Al-Assad to Cannibals.
As at the G20 summit in 2009, Obama is due to hold private talks with his Russian counterpart, with the American leader set to speak to Putin on the sidelines of the Belfast conference.