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."
Andrew Parker will take up his new role on Monday, April 22. Cabinet Secretary, Sir Jeremy Heywood, said:
"He brings his deep knowledge and experience of all aspects of the Service and its operations to this role and will be a great leader of the Service, working collaboratively with its partners. I look forward to working with him in his new role.
"I would also like to pay tribute to the outstanding way in which Jonathan Evans has led the Service since 2007 and I wish him all the very best for the future."
Under his [Andrew Parker's] leadership the Service will continue to stay ahead of global and domestic threats to our national security and further develop its reputation as one of the world's most effective security agencies.
I also pay tribute to the work of Sir Jonathan Evans, who has led the Service through challenging times of change and unrest, including in the aftermath of the 7/7 London bombings. His tireless work also helped ensure the delivery of a safe and successful Olympic and Paralympic Games last year.