By Gordon Corera
BBC security correspondent
In recent years, the notion of al-Qaeda as a decentralised organisation - a group that inspires attacks rather than organising and planning them itself - has gained common currency.
Security officials believe the al-Qaeda leadership has regrouped
But now senior counter-terrorist officials have told the BBC that they have seen evidence that this is no longer the case and that the threat has evolved.
It is believed that al-Qaeda has in fact regrouped in the Pakistan-Afghanistan border region and now poses a more direct threat, particularly to the UK.
Its communication channels and training facilities, which were heavily disrupted after the fall of the Taleban in Afghanistan, have been rebuilt and it is once more able to recruit members, communicate internationally and direct attacks.
Officials also believe that the close connections between communities in the UK and Pakistan mean that even though al-Qaeda would also like to attack the United States and other countries, the UK is likely to be the primary target because of the volume of travel and contact between the two countries.
New recruits are carefully selected and go through a process of indoctrination
There were 400,000 visits by UK residents to Pakistan in 2004 and a very small number of those are thought to have involved trips to training camps linked to al-Qaeda.
New recruits are carefully selected and go through a process of indoctrination.
They go on to form organised, self-contained cells which bear some similarities in their structure to those formed by the Provisional IRA in the past.
What concerns the authorities is the ease with which those who either drop out or are arrested can be replaced by other willing recruits
Different individuals will have different functions, often with one person acting as an overall leader and another in charge of getting hold of weapons or bomb-making.
In addition, there will be a number of foot-soldiers.
What concerns the authorities is the ease with which those who either drop out or are arrested can be replaced by other willing recruits.
The cat and mouse game between the authorities and these groups is also becoming increasingly complex.
The cells are increasingly aware of the attempts by authorities to conduct surveillance activity and are becoming increasingly sophisticated in their attempts to evade eavesdropping.
This often involves conducting conversations in public spaces and other methods, again used by Irish Republican groups in the past.
Both the volume of activity and its increased sophistication has made officials worry that the 7/7 bombings last year may simply have been the start of what could be a prolonged campaign.