The second woman to head MI5 takes over as the director-general today with the domestic security agency facing one of the most difficult periods in its history.

Eliza Manningham-Buller succeeds Sir Stephen Lander, who stepped down this year. The only other female leader of MI5 has been Stella Rimington, whose tenure created some controversy.

Mrs Manningham-Buller is the 52-year-old daughter of a former conservative attorney-general and a schoolmate of Princess Anne at Benenden.

Her appointment comes at a time when MI5 is striving for more public transparency and accountability.

In the week Mrs Manningham-Buller begins her new job, the service looks set to face some of its old problems with the start of the trial of David Shayler at the Old Bailey.

Mrs Manningham-Buller, who joined MI5 in 1974 after working for three years as a teacher, played a senior role investigating the Lockerbie bombing and was also one of the handlers of the senior KGB defector Oleg Gordievsky.

In 1992, she became the head of the newly-created counter-terrorism section for Northern Ireland.

Mrs Manningham-Buller was appointed deputy director at the same time.

Her husband, David, is the son of a former lieutenant-colonel and a former lecturer in moral philosophy at St Andrews University. He has recently retrained as a carpenter.

An Irish Catholic by birth, he is said to have once held strong left-wing views. According to friends, he did not know his future wife's profession during their courtship.