British intelligence services intercepted communications by delegates at two G20 summits in 2009 in London, the Guardian newspaper reported.
Computers and telephones were monitored, the newspaper reported online late on Sunday, citing the former US intelligence official Edward Snowden and documents seen by its reporters.
Some delegates were led to use internet cafes that had been set up to enable their messages to be tracked by Britain’s Government Communications Headquarters, the communications arm of the intelligence services.
The initiative, which targeted countries including Turkey and South Africa, was to improve Britain’s negotiating position during the summit, rather than address security concerns, the report said.
The Guardian said the revelations were likely to cause tensions among the delegates attending the G8 summit in Northern Ireland, which starts on Monday.