The new director general of MI5 has been unveiled as the intelligence agency's current deputy Andrew Parker.
Mr Parker, who has been with the Security Service since the early 1980s, will replace Sir Jonathan Evans on April 22.
Mr Parker, who led the agency's response to the July 7 attacks on London in 2005 as director of international terrorism, said: "It's a great honour to be appointed director general of MI5. I'm extremely proud of the extraordinary work the men and women of MI5 do to keep the country safe in challenging circumstances.
"I look forward to leading the service through its next chapter."
Mr Parker, 50, who has been in his current role as deputy director general since 2007, has held positions in Middle East terrorism, counter espionage, Northern Ireland terrorism, serious and organised crime, protective security, as well as policy and strategic planning.
The married father-of-two spent three years on secondment to HM Customs & Excise as director of intelligence before returning to the Security Service in 2002 to join the board as director for Northern Ireland terrorism, protective security and serious crime.
In 2006, as director of international terrorism, his teams played the lead role in the disruption of al Qaida's attempt to attack multiple airliners with bombs hidden in drinks bottles.
Mr Parker, who holds a degree in Natural Sciences from Cambridge University, takes up the top role at the agency at a time when the threat of terrorism is diversifying, coming from new areas such as Syria and North Africa. The withdrawal of troops from Afghanistan in 2014 is also likely to pose new challenges.
Home Secretary Theresa May said: "Under his 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."