The US National Security Agency has been granted a patent for technology that can provide the rough physical location of internet users based on their IP address.

Granted last month, the patent application outlines how the geographic location of internet users could be used to "measure the effectiveness of advertising across geographic regions" or flag a password that "could be noted or disabled if not used from or near the appropriate location".

The technology appears to be based on measuring the 'latency' (the time lag between computers exchanging data) of "numerous" locations on the internet and building a "network latency topology map".

The location is then estimated based on how long it takes a known computer to connect to the unknown one.

But the system does not work for dial-up connections, users of which can only be traced to their internet service provider.

The technology is known generically as 'geo-targetting' and is already used by online advertisers to target ads at specific regions.

"This is interesting but it is hard to see a commercial application at the moment as it may be very expensive to use," said Andy Clark, director at IT forensic firm Inforenz. "It also depends on using the Whois database and relies on that being up to date."

The Whois database is held by the Internet Corporation for assigned Names and Numbers and is updated by ISPs.

Currently lawyers must apply to the courts to gain a warrant to request an ISP to retrieve records that give details on users of accounts.