The Director General of MI5 has defended the security services right to remain secret in the wake of the Edward Snowden NSA and GCHQ leaks.
Speaking in public for the first time since taking on the role Andrew Parker said publishing details of the service's work, "hands the advantage to the terrorists."
He said: "What we know about the terrorists, and the detail of the capabilities we use against them together represent our margin of advantage. That margin gives us the prospect of being able to detect their plots and stop them.
"But that margin is under attack. We are facing an international threat and GCHQ provides many of the intelligence leads upon which we rely.
"It makes a vital contribution to most of our high priority investigations. It causes enormous damage to make public the reach and limits of GCHQ techniques.
"Such information hands the advantage to the terrorists. It is the gift they need to evade us and strike at will. Unfashionable as it might seem, that is why we must keep secrets secret, and why not doing so causes such harm."