Terrorists could get off scot-free simply because intelligence agencies are unable to intercept all forms of modern communications, according to the head of MI5.
Jonathan-Evans warned that some of his agents had expressed concern that their task of monitoring suspects was being impaired because some newer forms of communication were inaccessible to them.
The comments are likely to reignite a wider debate in government over its moves to give the security services enhanced powers to monitor the email, phone calls and internet history of suspects.
Such measures could cost the taxpayer as much as £5bn a year to run according to telecoms firms and prominent opponents include the deputy prime minister Nick Clegg.
The draft Communications Bill could force telecoms giants to keep a log of their customers activities, not including their content, for as much as a year.
In a report to MPs Evans said: “In general, it is becoming more difficult to be confident that you are getting CD [communications data] coverage of the targets that we need to look at. Therefore, from our point of view, the ability to go some way at least to future-proofing our access to CD is very important.
“If you are trying to pursue the activities of an individual or a group of individuals who are actively seeking to undertake a terrorist attack at some time in the future, then the ability to keep up with them in real time is absolutely critical.”