British intelligence agencies intercepted the communications of foreign politicians in London for the 2009 G20 summit, the Guardian reports.
The details were revealed in documents uncovered by NSA whistleblower Edward Snowden.
The Guardian reports the documents reveal that during meetings in April and September 2009, the GCHQ (Government Communications Headquarters) used "ground-breaking intelligence capabilities" to intercept the communications of delegates.
Methods included tricking delegates into using fake internet cafes so spy agencies could read their emails and hacking the delegates' BlackBerrys to allow emails and phone calls to be monitored.
British allies Turkey and South Africa were among those targeted.
The newspaper is also reporting that former Russian president Dmitry Medvedev was spied on by UK-based US agents at the same G20 summit.
The details of the intercept are laid out in a National Security Agency briefing which was shared with high-ranking officials from New Zealand, Britain, Australia and Canada.
Britain is about to begin hosting the G8 summit later today.