MI5 deputy Jonathan Evans is due to take over as head of the UK national security service.
Mr Evans has been deputy director since 2005
Mr Evans, 49, is to succeed Dame Eliza Manningham-Buller on Saturday, who is retiring after five years in the job.
As MI5's 16th director general, Mr Evans will be responsible for leading the battle against the service's current main focus on terrorism.
Home Secretary John Reid paid tribute to the outgoing head's "truly remarkable" 33-year career.
Dame Eliza recently spoke of the continuing terror threat facing Britain.
MI5 has increased in size by nearly 50% since the September 11 2001 attacks in the US and now has around 2,800 staff.
"Her determined attitude has, without doubt, helped the security service to develop and meet the challenges it faces in the 21st Century, providing strong leadership through times of crisis," said Mr Reid.
The new director general has been in the security service since 1980, specialising in counter-terrorism.
In the late 1980s and 1990s Mr Evans spearheaded the fight against Irish republican terrorism and was later involved in combating international threats and Islamic extremism.
In 2001, 10 days before the 9/11 attacks, he joined MI5's management board as director of the international counter-terrorism section G-branch.
He also helped implement policy changes as part of a modernisation of the service.
He was appointed deputy director general in 2005.
Crispin Black a security expert and former intelligence officer, said Mr Evans' was well qualified for the top job.
"His CV is really impeccable," he said.
"He was part of the reorganisation of MI5 in the 1980s so he knows how to reorganise things," he said.