The head of the GCHQ electronic intelligence gathering agency is to stand down, the Foreign Office has said.
Sir Iain Lobban, 53, is leaving after five and a half years in one of the most sensitive jobs in Whitehall, in what was said to be a long-planned move.
Officials denied any suggestion that his departure was linked to the recent controversy over the activities of GCHQ and its US counterpart, the NSA, sparked by the disclosures of the former US intelligence contractor, Edward Snowden.
He is expected to leave towards the end of the year but his decision to stand down is being made public now so that preparations to appoint a successor can be put in hand.
"Iain Lobban is doing an outstanding job as director GCHQ," a Foreign Office spokesman said. "Today is simply about starting the process of ensuring we have a suitable successor in place before he moves on as planned at the end of the year."
Sir Iain has already had one extension to his tenure as director since his appointment in June 2008. By the time he leaves, among the current crop of government department heads, only Sir Nicholas Macpherson at the Treasury will have served for longer.
ln November, he became the first head of the agency to give evidence in public when he appeared before the parliamentary Intelligence and Security Committee alongside the heads of MI5 and MI6.
At that hearing he warned that Mr Snowden's disclosures had done immense damage to Britain's counter-terrorism efforts. Critics however accuse him of presiding over a programme of mass intrusion into the private communications of millions of ordinary citizens.
Sir Iain has spent most of his working life at GCHQ having first joined the agency in 1983.
In Whitehall, he is credited with transforming the agency from a "distant cousin" - based out in Cheltenham - and putting it front and centre of the national security effort. In particular he is said to have foreseen the emerging threat of cyber attack.