But he said: “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.”

He said the details of what capabilities the spy agencies have is their “margin of advantage” over the fanatics.

“That margin gives us the prospect of being able to detect their plots and stop them. But that margin is under attack,” he said.