The NSA’s invention is intended automatically to sift through human speech transcripts in any language. The patent document specifically mentions “machine-transcribed speech” as a potential source.

Bruce Schneier, author of Applied Cryptography, a textbook on the science of keeping information secret, believes the NSA currently has the ability to use computers to transcribe voice conversations.

“One of the holy grails of the NSA is the ability automatically to search through voice traffic. They would have expended considerable effort on this capability, and this indicates it has been fruitful,” he said.

To date, it has been widely believed that while the NSA has the capability to conduct fully automated, mass electronic eavesdropping on e-mail, faxes and other written communications, it cannot do so on telephone calls.

While cautioning that it was difficult to tell how well the ideas in the patent worked in practice, Schneier said the technology could have far-reaching effects on the privacy of international phone calls.

“If it works well, the technology makes it possible for the NSA to harvest millions of telephone calls, looking for certain types of conversations,” he said.

“It’s easy to eavesdrop on any single phone call, but sifting through millions of phone calls looking for a particular conversation is difficult,” Schneier explained. “In terms of automatic surveillance, text is easier to search than speech. This patent brings the surveillance of speech closer to that of text.”

The NSA declined to comment on the patent. As a general policy, the agency never comments on its intelligence activities.

Yaman Akdeniz, director of Cyber-Rights & Cyber-Liberties UK, warned that with the new patent and a proposed AT&T and BT joint venture, which will allow US law enforcement agencies to tap the new communications network: “We might have a picture in which all British communications are monitored by the NSA.”

The revelation of the NSA’s patent is likely to cause tensions with the European Parliament. Over the past two years, the Parliament has commissioned several reports which examined whether the NSA has been using its electronic ears for commercial espionage, particularly in areas where US corporations compete with European and other companies.

The NSA relies on an international web of eavesdropping stations around the world, commonly known as Echelon, to listen into private international communications. The network emerged from a secret agreement signed after the Second World War between five nations including Australia, New Zealand, Canada, Britain and the US. Two of the NSA’s most important satellite listening stations are located in Europe, at Menwith Hill in Yorkshire and Bad Aibling in Germany.

Julian Assange, a cryptographer who moderates the online Australian discussion forum AUCRYPTO, found the new patent while investigating NSA capabilities.

“This patent should worry people. Everyone’s overseas phone calls are or may soon be tapped, transcribed and archived in the bowels of an unaccountable foreign spy agency,” he said.

One of the major barriers to using computers automatically to sift through voice communications on a large scale has been the inability of machines to “think” like humans when analysing the often imperfect computer transcriptions of voice conversations.

Commercial software that enables computers to transcribe spoken words into typed text is already on the market, but it usually requires the machine to spend time learning how to understand an individual voice in order to produce relatively error-free text. This makes such software impractical for a spy agency which might want automatically to transcribe and analyse telephone calls on a large scale.

It is also difficult for computers to analyse voice conversations because human speech often covers topics that are never actually spoken by name. According to the NSA patent application, “much of the information conveyed in speech is never actually spoken and… utterances are frequently less coherent than written language”.

US Patent number 5,937,422 reveals that the NSA has designed technology to overcome these barriers in two key ways. First, the patent includes an optional pre-processing step which cleans up text, much of which the agency appears to expect to draw from human conversations. The NSA’s “pre-processing” will remove what it calls “stutter phrases” associated with speech based on text.

Second, the patent uses a method by which a computer automatically assigns a label, or topic description, to raw data. If the method works well, this system could be far more powerful than traditional keyword searching used on many Internet search engines because it could pull up documents based on their meaning, not just their keywords.

Dr Brian Gladman, former MoD director of Strategic Electronic Communications, said that while he doubted the NSA had deployed the patented system yet, the new technology could become a “potent future threat” to privacy.

“If the technology does what it says ­ automatically finding and extracting the meaning in messages with reasonable accuracy ­ then it is way ahead of what is being done now,” he said.

The best way for people to protect their private communications was to use encryption, he said. Encryption software programs scramble data to prevent eavesdropping. “I’m afraid widespread interception is a fact of life and this is what makes encryption so important,” he said.

“The problem in the UK is that our government is working with the US to prevent UK citizens defending themselves using encryption,” he said, referring to the continuing use of export controls to hamper the widespread availability of encryption products.

The NSA’s current spy technology may be more advanced than methods described in the patent because the application is more than two years old. The US Patent Office approved the patent on 10 August this year, but the NSA originally lodged the application on 15 April 1997. The US Patent office keeps all applications secret until it issues a patent.