Documents leaked by US former spy Edward Snowden appeared to show Britain spied on G20 delegates during meetings in London back in 2009, Britain's Guardian newspaper reported on Monday.
Among the officials targetted were delegates from NATO ally Turkey and from South Africa.
Britain used "ground-breaking intelligence capabilities" to monitor communications between officials at the two meetings in April and September of 2009, the paper reported.
The revelations are likely to be an embarrassment to Britain, which is hosting the two-day G8 summit in Northern Ireland from Monday.
The paper cited documents it had seen concerning the work of Britain's Government Communications Headquarters (GCHQ), which handles signals intelligence.
According to the files, British spies tricked delegates into using specially prepared Internet cafes. Those cafes allowed the spies to intercept communications and monitor email messages and phone calls through delegates' BlackBerry devices.
GCHQ was also able to track when delegates were contacting each other and the agency targeted certain officials, including the Turkish finance minister, according documents shown to the newspaper.
They also singled out South African computers for special attention, according to one document.
The paper also said that GCHQ received reports from a US National Security Agency attempt to listen in as then Russian president, Dmitry Medvedev, made a call via satellite to Moscow.
The documents suggest that orders to gather intelligence on delegates came from a senior level within the government of then prime minister, Labour's Gordon Brown, said The Guardian.
Two documents explicitly mention information being passed on to ministers.
Snowden is hiding in Hong Kong and the United States has launched a criminal investigation after the former CIA technical assistant blew the lid on the NSA's vast electronic surveillance operation.
Leaders from the G8 nations meet in Lough Erne, Northern Ireland, on Monday for two days of talks on issues including the Syrian crisis, tax collection and free trade.