By Gordon Corera
BBC security correspondent
The MI5 head said future attacks could be chemical or nuclear
The day after the 7 July bombings, Eliza Manningham-Buller said she gave a talk to all the staff of MI5.
She reminded the assembled audience that it was always a matter of when and not if a terrorist attack took place and that they were in the privileged position of being able to make a difference.
MI5 has grown rapidly over recent years - increasing by 50% since 9/11 and it will have doubled in size by 2008. But so has its workload.
Since January of 2006, its casework on counter-terrorism has increased by 80%.
This is partly a result of finding more cases because it has more people who are looking harder - but it is also believed to be the result of more actual terrorist activity in the UK.
The scale of activity leads to hard choices. Every week, in co-ordination with the police, MI5 has to decide which of its many investigations it will prioritise and, every day, it has to make further decisions on how to apply its resources - whose phones to tap, who to follow.
It takes many officers to conduct 24-hours-a-day, seven-days-a-week surveillance so putting resources in one area involves diverting them from other investigations.
This, Dame Eliza acknowledged, is "risky" and getting it wrong - and a group who were not prioritised carrying out an attack - would lead to intense criticism.
It is rare for the director general of MI5 to speak out. And it is even rarer for her to provide such specific detail about the level of terrorist activity that her service faces.
The level of detail is clearly designed to emphasise that talk of an enduring, generation-long terrorist threat is not a matter of empty words but backed up by the reality of what MI5 is seeing on the ground.
MI5 has increased in size by nearly 50% since 9/11
Incidents like the Forest Gate raid have increased scepticism in some quarters about intelligence and Dame Eliza acknowledged that some people are reluctant to accept assertions that do not always appear to be substantiated.
But her hope is clearly that her speech on Thursday will convince people that a real problem exists and it is one which does not appear to have an end in sight.
MI5 may be working hard to disrupt the 200 groups and 30 plots that it sees, but radicalisation - especially among the young - means that many more people could take the place of those arrested.
One further problem is the speed of radicalisation which means individuals can very quickly move towards becoming involved in terrorism or move from facilitating others to planning their own attacks.