The head of MI5 is to leave her post after four and a half years.

Dame Eliza Manningham-Buller, who has been director general of the security service since October 2002, said she had been planning the move since before last year's terrorist attacks on the London Underground.

Her replacement will be appointed by the Home Secretary John Reid in the New Year.

The daughter of a former Conservative Lord Chancellor, Dame Eliza became only the second woman to head the service on her appointment. She came under focus following the suicide bombings on London's transport network in July last year and has spoken recently of the continued threat to UK security posed by terrorism.

Announcing her resignation yesterday, Dame Eliza said in a statement: "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 at a dramatic rate to tackle that threat.

advertisement"I'm confident that the service will continue to serve the UK to the best of its ability and I shall watch its progress with great interest."

Dame Eliza, 58, agreed her resignation with former Home Secretary Charles Clarke, who was sacked in May. Mr Reid said: "Eliza has given 33 years of dedicated service to the nation and we owe her a tremendous debt of gratitude."