The NSA analysed and stored phone, internet and email details of UK citizens under a secret pact approved by the British intelligence authorities, the latest leaked documents from whistleblower Edward Snowden claim.

According to an NSA memo cited by the Guardian and Channel 4, both countries entered an agreement in 2007 that gave the NSA access to personal data about British citizens – something which was earlier prohibited.

The memo declares: "Sigint (signals intelligence) policy… and the UK Liaison Office here at NSAW (NSA Washington) worked together to come up with a new policy that expands the use of incidentally collected unminimized UK data in Sigint analysis."

"The new policy expands the previous memo issued in 2004 that only allowed the unminimising of incidentally collected UK phone numbers for use in analysis.

"Now SID analysts can unminimise all incidentally collected UK contact identifiers, including IP (Internet protocol) and email addresses, fax and cell phone numbers, for use in analysis," memo added.

Snowden’s data revealed that the information had been ‘incidentally collected’ by the US intelligence agency, implying that the individuals were not the main targets of spy operations.

The US agency had been making use of the UK information to carry out ‘pattern of life’ or ‘contact-chaining’ analyses, which allowed the agency to search for up to three ‘hops’ away from a target including the communications of a friend of a friend of a friend.