By Ewen MacAskill, Nick Davies, Nick Hopkins, Julian Borger and James Ball

American spies based in the UK intercepted the top-secret communications of the then Russian president, Dmitry Medvedev, during his visit to Britain for the G20 summit in London, leaked documents reveal.

The details of the intercept were set out in a briefing prepared by the National Security Agency (NSA), America’s biggest surveillance and eavesdropping organisation, and shared with high-ranking officials from Britain, Australia, Canada and New Zealand.

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The document, leaked by the NSA whistleblower Edward Snowden and seen by the Guardian, shows the agency believed it might have discovered “a change in the way Russian leadership signals have been normally transmitted”.

The disclosure underlines the importance of the US spy hub at RAF Menwith Hill in Harrogate, North Yorkshire, where hundreds of NSA analysts are based, working alongside liaison officers from GCHQ.

The document was drafted in August 2009, four months after the visit by Medvedev, who joined other world leaders in London, including the US president, Barack Obama, for the event hosted by the British prime minister, Gordon Brown.

Medvedev arrived in London on Wednesday 1 April and the NSA intercepted communications from his delegation the same day, according to the NSA paper, entitled: “Russian Leadership Communications in support of President Dmitry Medvedev at the G20 summit in London – Intercept at Menwith Hill station.”

The document starts with two pictures of Medvedev smiling for the world’s media alongside Brown and Obama in bilateral discussions before the main summit.

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The report says: “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.”

The NSA interception of the Russian leadership at G20 came hours after Obama and Medvedev had met for the first time. Relations between the two leaders had been smoothed in the runup to the summit with a series of phone calls and letters, with both men wanting to establish a trusting relationship to discuss the ongoing banking crisis and nuclear disarmament.

In the aftermath of their discussions on 1 April, the two men issued a joint communique saying they intended to “move further along the path of reducing and limiting strategic offensive arms in accordance with the treaty on the non-proliferation of nuclear weapons”.

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A White House official who briefed journalists described the meeting as “a very successful first meeting focused on real issues”. The official said it had been important for the men to be open about the issues on which they agreed and disagreed. Obama had stressed the need to be candid, the official noted.

While it has been widely known the two countries spy on each other, it is rare for either to be caught in the act; the latest disclosures will also be deeply embarrassing for the White House as Obama prepares to meet Vladimir Putin, who succeeded Medvedev as president, in the margins of the G8 summit this week.

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The two countries have long complained about the extent of each other’s espionage activities, and tit-for-tat expulsions of diplomats are common. A year after Obama met Medvedev, the US claimed it had broken a highly sophisticated spy ring that carried out “deep cover” assignments in the US.

Ten alleged Russian spies living in America were arrested.

Putin was withering of the FBI-led operation: “I see that your police have let themselves go and put some people in jail, but I guess that is their job. I hope the positive trend that we have seen develop in our bilateral relations recently will not be harmed by these events.” Last month, the Russians arrested an American in Moscow who they alleged was a CIA agent.

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The new revelations underline the significance of RAF Menwith Hill and raise questions about its relationship to the British intelligence agencies, and who is responsible for overseeing it. The 560-acre site was leased to the Americans in 1954 and the NSA has had a large presence there since 1966.

It has often been described as the biggest surveillance and interception facility in the world, and has 33 distinct white “radomes” that house satellite dishes. A US base in all but name, it has British intelligence analysts seconded to work alongside NSA colleagues, though it is unclear how the two agencies obtain and share intelligence – and under whose legal authority they are working under.