Sir John Sawers will take up his post early in November
Britain's ambassador to the United Nations is to become the new chief of MI6, Downing Street has announced.
Sir John Sawers, 52, will head the overseas Secret Intelligence Service in November, replacing Sir John Scarlett.
The departing boss spent more than five years in the post, having switched from his role leading the Joint Intelligence Committee just before the Iraq war.
Prime Minister Gordon Brown's spokesman said the departing Sir John had done "an important and valuable job".
He had made a real contribution to protecting Britain from international terrorism and other global threats, the spokesman added.
He also said Sir John Scarlett's move was in no way connected to the impending inquiry into the Iraq war, at which he is likely to be a key witness.
Foreign Secretary David Miliband welcomed the appointment, stressing the importance of the role.
"International terrorism, the proliferation of weapons of mass destruction and the growth of regional conflict and instability mean that there has never been a time when the skills and dedication of our intelligence agencies, including SIS, have been more necessary to our national security and the safety of our people around the world," he said.
Alluding to the time when the chief of the SIS would have been known as 'C' to keep his identity secret, Mr Miliband said "I take this opportunity to pay tribute to Sir John Scarlett's outstanding record as 'C' over more than five years in the post.
"I look forward to a continued close working relationship with him over the coming months."
BBC Security Correspondent Frank Gardner says the appointment is a sign of how far MI6 has come into the open.
He said Sir John Sawers was a highly recognisable figure and a career diplomat, unlike the incumbent Sir John Scarlett who played a key role in espionage against the Soviets.
The incoming boss has been Britain's Permanent Representative to the UN since 2007.
Before that, he was political director at the Foreign Office, an envoy in Baghdad and also a foreign affairs adviser to ex-prime minister Tony Blair.
In the latter post from 1999 to 2001 he was involved in the Kosovo conflict and Northern Ireland peace process.
Elsewhere overseas, he worked in the British Embassy in Washington, as an ambassador to Cairo and in South Africa from 1988 and 1991 as apartheid was ending.
Downing Street also said Sir John was re-joining the intelligence services but would not give any further details.
The new boss is married to teacher Shelley and has three grown-up children.
His replacement as UN ambassador has yet to be announced.