There have been new allegations that the US National Security Agency was given permission by the British government to spy on British citizens, even if they were not suspected of criminal activity.
British media are reporting that a leaked NSA memo described a change in the rules six years ago.
Under a seven decade-old British-United States agreement on intelligence sharing, Britain and American spy agencies were not supposed to collect information on each other without permission, the BBC reports.
But according to The Guardian newspaper and Channel 4 News, in 2007 existing rules were modified to allow the US to obtain more information about British communications which it had collected incidentally as part of its global communications gathering operations.
Previously, anything other than landline phone numbers could not be kept, but after 2007 mobile phone numbers and email addresses could be retained.
The reports are based on documents leaked by US fugitive whistleblower Edward Snowden, who worked in intelligence.
The documents say after the rules changed, the US was able to keep the records to analyse them for the specific purpose of "contact chaining", so as to look at connections.
Britain's Foreign Office said it does not comment on such "speculation".