By Gordon Corera
BBC security correspondent
The terrorist threat is evolving rapidly and has not yet peaked. That was the message from the new director general of MI5 Jonathan Evans in his first speech.
MI5 is trying to adapt to cope with changing challenges
The threat has evolved in a number of ways in the last year since Evans' predecessor Eliza Manningham-Buller gave a dramatic speech in London outlining the scale of the threat.
One manifestation of that change is the way in which al-Qaeda conspiracies in the UK are being driven from an increasing range of overseas countries.
The threat emanating from core al-Qaeda based in the tribal areas on the Pakistan and Afghanistan border remains the number one concern, senior British counter-terrorism officials say.
People continue to travel there for training - often going to great lengths to make the journey.
But there is now increasing evidence that other regions are also the source of training and of planning attacks against the UK.
Evans warns that al-Qaeda in Iraq is aspiring to promote terrorist attacks outside of the country.
Other al-Qaeda 'franchises' also pose a growing threat, including al-Qaeda in East Africa which has been using Somalia as a base for training and planning including against the UK.
Other senior counter-terrorism officials also talk of countries like Nigeria and Bangladesh being of increasing concern and say that the source of potential threat within the UK increasingly comes not just from ethnic communities with links back to Pakistan, but from groups with other backgrounds.
The emergence of an al-Qaeda franchise in North Africa, centred on Algeria, is of particular concern to some European countries like France although so far it has shown less sign of targeting the UK.
Across Europe, Evans noted, the last 12 months have seen an increase in attack planning although he says it is too early to assess with confidence what exactly this means.
The range of people becoming involved in the UK is also changing, with young people increasingly being targeted - some as young as 15 or 16.
The old idea that radicalisation was centred on mosques has now evolved to one in which the process is seen as often taking place within loose social networks of friends.
Young people themselves are often radicalising others, using youth clubs or the internet.
Identifying those within communities who are engaged in radicalising and recruitment is a priority but not always easy, according to officials.
Jonathan Evans took over as director of MI5 in April
Evans makes clear that MI5 is adapting to try and cope with these changes by growing in size, moving much more of its work into the regions as well as by co-operating with other partners both within the UK and internationally.
MI5 has grown rapidly in recent years with a target of 4,000 staff by 2011 but with at least 2,000 individuals believed to pose a threat to national security, basic maths says that the service has to prioritise among those targets because it is impossible to keep tabs on all of them all the time.
"Every decision by the security service to investigate someone entails a decision not to investigate someone else," he said.
MI5 was criticised for not investigating Mohammed Siddique Khan who went on to kill on 7 July 2005 after he emerged in another counter-terrorist investigation.
Evans was keen to emphasise that this is likely to happen again in future cases because of the sheer numbers and because of the way in which extremists know each other - they do not operate in the small, compartmentalised way that other terrorist groups have.
This could lead to questions once again in the wake of an attack but does offer advantages in terms of tracing individuals and their connections faster.
Evans also brought up the issue of prioritisation and resources in a surprisingly direct criticism of Moscow, saying that Russian covert activity in the UK meant that resources which could be directed against terrorism were instead being used to keep tabs on Russian intelligence officers.
Evans also stressed that terrorist attacks are the visible manifestations of a deeper problem whose root is ideological and that combating the ideology requires a much broader strategic effort across government.
Whitehall officials concede that of the government's counter-terrorist strategy, the area which has been struggling most is that focused on preventing terrorism by dealing with radicalisations.
However successful MI5 and the police may be at pursuing existing terrorists, the real challenge is preventing more individuals joining their cause - and especially the young.