Ex-spy chief Sir David Pepper has told BBC Two programme Who's Watching You? the web presents an unprecedented challenge for the intelligence services.
In his first broadcast interview, the former director of the listening service GCHQ says it has a major bearing on the way suspected terrorists are monitored.
Special correspondent Richard Bilton reports.
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Who's Watching You
The three part series Who's Watching You? concludes on Monday 8 June, 2009 at 2100 BST on BBC Two. Or catch-up with episodes one and two anytime on BBC