The new director general of the domestic security service, known as MI5, is to be the current deputy, Andrew Parker.
Mr Parker will take over from the outgoing head, Sir Jonathan Evans, next month.
Mr Parker led the security services' response to the 2005 London attacks.
Announcing the appointment, Home Secretary Theresa May said Mr Parker had a "wealth of experience and knowledge".
Mr Parker, 50, a keen ornithologist, has worked for MI5 for 30 years. He has worked on terrorism in the Middle East and Northern Ireland, counter espionage, and serious and organised crime.
He oversaw an expansion in counter terrorism capability and the development of the service's regional network.
In 2006, his teams played the lead role in the disruption of Al-Qaeda's attempt to attack multiple airliners with bombs hidden in drinks bottles.
He has been the deputy director general since 2007.
He said: "It is a great honour to be appointed Director General of MI5.
"I am extremely proud of the extraordinary work that 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."
Ms May added: "I am very pleased to announce the appointment of Andrew Parker as the new DG of the Security Service, a role to which he brings a wealth of experience and knowledge.
"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."
Sir Jonathan's departure was announced last week.
He succeeded Lady Eliza Manningham-Buller to become head of MI5 in April 2007.
He will leave at the end of his contract, having spent 33 years with the security service.
Sir Jonathan's work included investigating counter espionage, developing and implementing key policies on security and most-recently countering the threat of international terrorism.
At MI5, he initially focused on Irish-related terrorism in the 1980s and 1990s, before moving on to international counter-terrorism investigations in 1999.
He was appointed director of international counter-terrorism just 10 days before the 9/11 attacks on the World Trade Center and the Pentagon in 2001.