The cover has been blown off one of the NSA's covert programs. Named XKeyscore, the program covers emails, social media activity and browsing history and is accessible to NSA analysts with little or no prior authorization, according to a leaked presentation published by The Guardian today.

The Guardian reports that the top secret National Security Agency program allows analysts to search through a database "containing emails, online chats and the browsing histories of millions of individuals". In the leaked documents, the NSA describes XKeyscore as its "widest-reaching" internet intelligence system.

Despite the short shelf life of data stored on XKeyscore in one month last year, the system collected at least 41 billion total records.

In a statement to The Guardian, the NSA said: "NSA's activities are focused and specifically deployed against – and only against – legitimate foreign intelligence targets in response to requirements that our leaders need for information necessary to protect our nation and its interests.

"XKeyscore is used as a part of NSA's lawful foreign signals intelligence collection system.

"Allegations of widespread, unchecked analyst access to NSA collection data are simply not true. Access to XKeyscore, as well as all of NSA's analytic tools, is limited to only those personnel who require access for their assigned tasks … In addition, there are multiple technical, manual and supervisory checks and balances within the system to prevent deliberate misuse from occurring."

The NSA statement continues: "Every search by an NSA analyst is fully auditable, to ensure that they are proper and within the law.

"These types of programs allow us to collect the information that enables us to perform our missions successfully – to defend the nation and to protect US and allied troops abroad."