Among the officials targeted were delegates from Nato ally Turkey and from fellow Commonwealth state South Africa, said The Guardian.
Britain used “ground-breaking intelligence capabilities” to monitor communications between officials at the two meetings in April and Sept 2009, the paper reported.
British prime minister David Cameron refused to comment on the report.
Asked whether he could guarantee his guests that no similar operation was in place as they gathered at the luxury Lough Erne resort, Cameron would not be drawn.
“We never comment on security or intelligence issues and I am not about to start now,” he said.
“I don’t make comments on security or intelligence issues — that would be breaking something that no government has previously done.”
The Guardian cited documents it had seen on the work of Britain’s Government Communications Headquarters, its electronic eavesdropping agency.
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 to documents shown to The Guardian.
They also singled out South African computers for special attention, according to one document.
They were said to have enabled a team of 45 analysts to receive live round-the-clock summaries of who was phoning whom during the proceedings.
The Guardian also said GCHQ received reports from a US National Security Agency attempt to listen in as Dmitry Medvedev, then the Russian president, 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 Britain’s then prime minister Gordon Brown, said The Guardian.
Two documents explicitly mention information being passed on to ministers.
A briefing paper prepared for GCHQ director Iain Lobban, dated Jan 9, 2009, set out the government’s priorities for the April G20 leaders’ summit.
“The GCHQ intent is to ensure that intelligence relevant to Her Majesty’s government’s desired outcomes for its presidency of the G20 reaches customers at the right time and in a form which allows them to make full use of it,” it noted.
A week after the September summit of finance ministers, an internal review concluded: “The call records activity pilot was very successful and was well-received as a current indicator of delegate activity.
“It proved useful to note which national delegation was active during the moments before, during, and after the summit. All in all, a very successful weekend with the delegation telephony plot.”
Snowden is hiding in Hong Kong. The US has launched a criminal investigation after the former CIA technical assistant blew the lid on the NSA’s vast electronic surveillance operation.
The Guardian and The Washington Post earlier this month published leaked information from the 29-year-old intelligence technician that revealed the existence of a top-secret NSA programme to collect and analyse data from internet users around the world.
Meanwhile, Apple says it received between 4,000 and 5,000 requests from US law enforcement for customer data for the six months ended in May.
The company, like some other businesses, had asked the US government to be able to share how many requests it received related to national security and how it handled them. Those requests were made as part of Prism, the recently revealed highly classified NSA program that seizes records from internet companies.
Prism appears to do what its name suggests. Like a triangular piece of glass, Prism takes large beams of data and helps the government find discrete, manageable strands of information.
Prism touched off the latest round in a decade-long debate over what limits to impose on government eavesdropping, which the Obama administration says is essential to keep the nation safe.
Apple said that between 9,000 and 10,000 accounts or devices were specified in data requests between Dec 1, 2012, and May 31 from federal, state, and local authorities and included both criminal investigations and national security matters.
It said the most common form of request came from police investigating robberies and other crimes, searching for missing children, trying to locate a patient with Alzheimer’s disease, or hoping to prevent a suicide.