The new head of the security service, known as MI5, is to be its current deputy, Jonathan Evans.
Mr Evans' focus has been counter terrorism
Mr Evans, 49, will take over from Director General Dame Eliza Manningham-Buller who retires next month, the Home Office said.
He has been in the security service since 1980, specialising in counter-terrorism.
Dame Eliza warned last year that the UK faced a "serious" and "growing" threat from terrorism.
As head of MI5, Mr Evans will be responsible for leading the battle against terrorism within the UK.
In the late 1980s and 1990s he spearheaded the fight against Irish republican terrorism.
Since 1999, he has been directly involved in combating the threat from Islamic extremism and international terrorism.
In 2001 he was appointed to the security service's management board as director of international counter terrorism - 10 days before the 9/11 attacks.
He became deputy director general of MI5 in 2005.
Before announcing her planned departure, Dame Eliza said there were 30 terror plots threatening the UK and MI5 was keeping 1,600 individuals under surveillance.
Dame Eliza warned the threat was "growing" in her speech in December 2006.
And she outlined the challenges that will be faced by her successor.
She said future attacks could be chemical or nuclear and that many of the plots were linked to al-Qaeda.
"Today we see the use of home-made improvised explosive devices," she said.
"Tomorrow's threat may - I suggest will - include the use of chemicals, bacteriological agents, radioactive materials and even nuclear technology."
MI5 has increased in size by nearly 50% since the September 11 attacks in the US and now has around 2,800 staff.