The Turkish finance minister Mehmet Simsek, together with possibly 15 others in his party, was said to have been targeted.
Intelligence agencies also received reports from an NSA attempt to eavesdrop on the Russian leader, Dmitry Medvedev, as his phone calls passed through satellite links to Moscow, it was reported.
A total of 45 analysts are said to have been supplied with a live round-the-clock summary of who was phoning who at the summit.
The documents allegedly suggest that the operation was sanctioned in principle at a senior level in the government of the then prime minister, Gordon Brown, and that intelligence, including briefings for visiting delegates, was passed to British ministers.
The news comes as the UK prepares to host the G8 summit on Monday, attended by all of the nations that were at the 2009 meetings which were reportedly the target of systematic spying.
The allegations could lead to tension among visiting delegates seeking answers over whether they were spied upon in the London G20 meetings, and whether they will be subjected to similar measures this week.
The G20 spying allegedly appears to have been organised for the purpose of securing an advantage in meetings, with named targets said to include long-standing allies such as South Africa and Turkey.
There have often been rumours of this kind of espionage at international conferences, but it is unusual for evidence to confirm it and give details, it was reported.
A GCHQ spokesman said: "We do not comment on security matters."
Intelligence agencies also received reports from an NSA attempt to eavesdrop on the Russian leader, Dmitry Medvedev, as his phone calls passed through satellite links to Moscow, it was reported.
A total of 45 analysts are said to have been supplied with a live round-the-clock summary of who was phoning who at the summit.
The documents allegedly suggest that the operation was sanctioned in principle at a senior level in the government of the then prime minister, Gordon Brown, and that intelligence, including briefings for visiting delegates, was passed to British ministers.
The news comes as the UK prepares to host the G8 summit on Monday, attended by all of the nations that were at the 2009 meetings which were reportedly the target of systematic spying.
The allegations could lead to tension among visiting delegates seeking answers over whether they were spied upon in the London G20 meetings, and whether they will be subjected to similar measures this week.
The G20 spying allegedly appears to have been organised for the purpose of securing an advantage in meetings, with named targets said to include long-standing allies such as South Africa and Turkey.
There have often been rumours of this kind of espionage at international conferences, but it is unusual for evidence to confirm it and give details, it was reported.
A GCHQ spokesman said: "We do not comment on security matters."