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It concludes: “It is better to be discreet when searching on the internet rather than employ deception.”

On both sides of the Atlantic, snooping on thousands of websites is being criticised by privacy campaigners.

In America there was a storm of controversy over the extent of the government’s tracking of ostensibly private information, including people’s blogs, and also details of telephone records and spending habits.

Simon Davies, director of the pressure group Privacy International, said: “People are starting to put very intimate details about themselves and other people online. It’s fertile territory for intelligence agencies.

“You can convince people to reveal information they never normally would, and you can extract information about their friends and associates.”

Britain has long experience of gathering open source information. In 1939 the government formed the BBC monitoring service which sifts newspapers, radio and television broadcasts from around the world.

Mark Lowenthal, an assistant director of the CIA until earlier this year, said open source information had long been undervalued. “We’re playing a lot of catch-up,” he said.

Professor Michael Batty, of University College London, said advances in analytical methods were increasing the available intelligence. “Connecting databases which traditionally have not been connected can provide enormous amounts of information,” he said.

Private organisations are also supplying detailed analysis of open source information. The SITE Institute in Washington, founded by Rita Katz, analyses “corporate records, tax forms, credit reports, video tapes, internet news group postings and owned websites, among other resources, for indicators of illicit activity”.

Katz, who was born in Iraq and speaks fluent Arabic, spends hours each day monitoring the password-protected online chatrooms in which Islamic terrorists discuss politics and pass on tips — how to disperse botulinum toxin or transfer funds, or which suicide vest is best.

She can identify potential suicide bombers by tracking when they announce they will be surrendering their online user names to become martyrs.

“It is completely addictive. You wake up thinking, I’ve been offline for several hours but the terrorists have been making plans,” she said.

Victory in the secret internet war may be some way off for the intelligence agencies, however.

The director of one firm providing information to the British government said: “The bad guys do open source intelligence as well. It’s amazing how much you can find out about what the Americans are buying and what their capabilities are.”

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