John Sawers will take up his new post in November, the prime minister's office said in a statement Tuesday.
He replaces John Scarlett, whose career has been dogged by concerns over the intelligence used to build the case for the 2003 U.S.-led invasion of Iraq. Scarlett is retiring after five years.
Prime Minister Gordon Brown's Downing Street office said in a statement that Sawers, 53, was rejoining MI6 — but declined to give details of his previous appointment with the intelligence agency, formally known as the Secret Intelligence Service or SIS.
MI6 only began publicly naming its chief in the 1990s. Until 1992, Britain's government refused to confirm the organization's existence.
Like all heads of MI6, Sawers will be known as "C" in tribute to the agency's first chief Capt. Mansfield Cumming — who signed himself "C" in green ink on official documents. Each chief since has used the initial "C" to sign off papers.
Sawers previously served as ex-Prime Minister Tony Blair's chief foreign policy adviser and as the Foreign Office's political director, when he joined negotiations with Tehran over its alleged pursuit of nuclear weapons.
He has also held diplomatic postings to New York, Washington, Syria, Yemen and South Africa. Sawers studied in Britain and at Harvard and the University of Witwatersrand in South Africa.
Sawers is currently the U.K.'s permanent representative to the U.N., leading the British delegation at the assembly.
British Foreign Secretary David Miliband said Sawers would lead the agency amid increasing threats from international terrorism, the proliferation of weapons of mass destruction.
"He's going to have his work cut out in terms of the deteriorating situations in both Iran and Pakistan," British opposition Conservative party lawmaker Patrick Mercer said.
Scarlett was head of Britain's Joint Intelligence Committee prior to his appointment as MI6 chief in May 2004.
He was called to give evidence to two government inquiries which examined intelligence gathered prior to the Iraq war — including one panel which criticized intelligence officials for relying on seriously flawed or unreliable sources.
Scarlett has overseen a rapid expansion of MI6 since the September 2001 attacks on the United States, and led the agency's work attempting to thwart attacks on Britain plotted overseas — chiefly in northwestern Pakistan.
Several major terrorism trials in recent years have seen Britons with family ties to Pakistan jailed. Brown has said that about two-thirds of all terrorist plots against Britain have links to Pakistan.
In recent testimony to Parliament's Intelligence and Security oversight committee, Scarlett acknowledged he was keeping veteran staff in their posts beyond their usual retirement age.
He said MI6 needed the knowledge and skills of aging staff amid the current terrorist threat.
