NSA surveillance programmes have the capacity to reach around three quarters of all US Internet traffic, with the agency routinely retaining the content of emails sent between US citizens inside the country.
The revelations come from an investigation by the Wall Street Journal, which spoke to a number of current and former US government and agency officials.
The 75 per cent figure is much higher than has been disclosed by the NSA. It has previously said only foreign communications are intercepted; indiscriminate spying on US citizens inside the country is illegal.
However, officials told the WSJ that, although the systems are designed to search for communications which originate or end abroad, or are entirely foreign but pass through domestic servers, the filter's far reach means that domestic communications are often intercepted.
It has also been revealed that PRISM is not the only programme which collects from Internet and telecoms companies, a variety of other programmes such as 'Blarney', 'Fairview', 'Oakstar', 'Lithium' and 'Stormbrew', all ensure that data is collected from every major US telecoms and Internet firms.
Through the interviews, the WSJ has been able to determine how the systems work. The first cut of the data is filtered by the companies - the NSA asks them to send over traffic which is most likely to include foreign communications.
The NSA then carried out the second cut . The agency copies all the traffic and through certain "strong selectors" decides what to keep. At this selection stage the NSA can already look at the content of messages, as well as who has sent and received them.
Through this system, together the companies and NSA cover around 75 per cent of all US Internet communications. Firms must cooperate with the agency due to legal orders from the secret Foreign Intelligence Surveillance Court.