Spies use the site to help them to “map” social networks and identify people who have come into contact with those who may pose a threat to national security.

The controversial initiative is part of a new secret programme to monitor thousands of internet blogs, bulletin boards and web chatrooms which the intelligence services are coming to regard as an “essential” source of information.

The Cabinet Office has formed the Open Source Joint Working Group (OSJWG) comprised of officers from MI5, MI6 and GCHQ to exploit the new “open sources” of intelligence. The group liases closely with a new American unit set up by the CIA.

Websites being scrutinised are thought to range from online “networking” sites aimed at teenagers such as Bebo and MySpace, to obscure blogs set up by Nepalese guerrilla groups. The spooks have even bought specialist “pod-mining” software to snoop on podcasts around the world.

A spokesman for the Foreign Office confirmed that Friends Reunited was among the sites being monitored: “There is obviously stuff that can be grabbed by anybody without intercepting communications. There is constant monitoring of sites for information which is open and can be of use.”

There are nearly 7m blogs in Britain alone, which have been doubling in number every few months in recent years. Those now running blogs include MPs’ researchers, policemen and squaddies serving on the frontline.

Sophisticated software programs are used to trawl the web looking for key words and people. Thousands of websites have been graded on their usefulness to the intelligence agencies. The information is then studied and “assessed” by intelligence analysts — who also manually watch sites of particular interest — from their base at the “knowledge centre” at GCHQ in Cheltenham.

Training manuals issued by Nato and used by intelligence agencies throughout the world reveal how spies can find information on the normal internet and on the so-called “deep web” — sites that have no links to others on the internet and therefore cannot be trawled by conventional search engines.

Nato has set up its own deep web search engine for use by agents working for member countries. One manual seen by The Sunday Times, entitled the Nato Open Source Intelligence Handbook, states: “Over 250,000 databases are now available within the deep web, a great many of potential intelligence value.”

The manuals reveal how spies can “lurk” and remain anonymous online. “It is quite possible to surf the web without openly identifying your identity, purpose or intentions,” says a handbook called Intelligence Exploitation of the Internet. “This is simply a case of ‘I won’t tell you unless you ask’.

“There may be occasions when you will not want others to know exactly who you are or who you work for . . . it is reasonably easy to create an anonymous persona on the web.”

Spies have also begun communicating with people in chatrooms to elicit information. But one Nato manual advises: “An anonymous persona should only be used for occasional requests for information. Any development of a relationship using the internet should be discouraged. This is in the field of other specialists, typically in the realm of Humint (human intelligence). Without proper control, such practices can lead to embarrassment.”

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