Dame Eliza Manningham-Buller is to quit as the head of MI5 after four and a half years in the job.
Dame Eliza, 58, who has been the director general of the Security Service since October 2002, said she had been planning the move since before last year's London terror attacks.
Her replacement will be appointed by the Home Secretary, John Reid, in the New Year.
"By April 2007 I shall have been an officer of the Security Service for 33 years, the last 10 as either deputy director general or director general," Dame Eliza said in a statement.
"I decided in early 2005 that it would be time by then to stand down.
"I have been privileged to lead the service when it is facing the two challenges of a very serious threat and the consequent need to grow and change to tackle that threat."Reuse content