MI5 chief Eliza Manningham-Buller says the "serious, growing threat" of terrorism will be "with us for a generation". So who is Dame Eliza?
Eliza Manningham-Buller has worked in intelligence for 32 years
During her time at the organisation's helm, she has been privy to some of Britain's most carefully guarded secrets and held the lives of spies and double agents in her hands.
During the early 1980s, only five people knew that Oleg Gordievsky, the deputy head of the KGB at the Soviet embassy in London, was actually a double agent.
Ms Manningham-Buller was one of them and, as Gordievsky later acknowledged, her ability to keep a secret saved his life.
Despite the fact that two of her assistants shared an office with Michael Bettany, a traitor working for the KGB, Gordievsky's crucial role was never mentioned.
One word in front of Bettany, later jailed for 18 years for spying for the Soviets, and Gordievsky would have been on the first plane to Moscow and an inevitable date with an executioner's bullet.
The MI5 chief taught Nigella Lawson during her brief teaching career
The MI5 chief was born the Honourable Elizabeth Lydia Manningham-Buller in 1948.
Her father, Sir Reginald Manningham-Buller (later Lord Dilhorne), served as Attorney General and Lord Chancellor in the Conservative administrations of Harold Macmillan and Sir Alec Douglas-Home.
She attended the exclusive Benenden girls' public school in Kent, where she was a contemporary of Princess Anne.
While there, her forthright character earned her the nickname "Bullying Manner".
She went on to read English at Lady Margaret Hall, Oxford, where it is believed MI5 first attempted to recruit her, much to her father's distress.
While at Oxford, she starred as the Fairy Godmother in a production of Cinderella directed by Giles Brandreth, now a former Tory MP, who later described her as "absolutely superb" in the role.
The show's choreographer, Michael Coveney, now a drama critic, described her as "imposing" and said she seemed older than her years.
After graduating, she worked as a teacher at the exclusive Queen's Gate school in London, where future journalist, celebrity chef and cookery writer Nigella Lawson was among her pupils.
There are many secrets at the MI5 headquarters in London
But, in 1974, she left teaching after three years to finally join MI5 at the height of the Cold War.
There, she soon progressed from typing up transcripts of tapped telephone conversations between Warsaw Pact diplomats to becoming a fully-fledged spycatcher.
An expert on counter-terrorism, she was heavily involved in the Lockerbie investigation, served as MI5's liaison officer in Washington and became director of the agency's Irish counter-terrorism branch, spearheading the fight against the Provisional IRA.
In 1997, she became MI5's deputy director general, taking on responsibility for the organisation's day-to-day work and relations its with other agencies at home and abroad.
Then, in 2002, she became only the second woman to head the service, taking over from Sir Stephen Lander.
His predecessor, and the only other woman to have held the post, Dame Stella Rimington, praised her on her appointment as a "highly intelligent, very experienced and very kind person".
Oleg Gordievsky's life was in Manningham-Buller's hands
But she warned that, as a woman, she would be subjected to more scrutiny about her clothes and appearance than a male chief.
A grateful Gordievsky said her appointment was "the best news for the service in a decade".
Others, however, said she was a "conservative" choice as secretary general, which showed MI5 was resistant to change.
Since taking over the MI5 reins, her working life has been dominated by the threat from al-Qaeda.
She has taken a higher profile than some of her predecessors, making several public speeches and issuing a stark public warning in June 2003 that it was "only a matter of time" before terrorists launched a biological, chemical or nuclear attack on a Western city.
She also broke with tradition by publishing information about terrorism risk assessments on the internet.
She was awarded the Dame Commander of the Order of the Bath (DCB) in the Queen's Birthday Honours in 2005.
Ms Manningham-Buller lives with her husband and his five children from an earlier marriage.
Despite her busy work life, she reportedly still insists on cooking a roast dinner for her family every Sunday.