Al-Qaeda has become more organised and sophisticated and has made Britain its top target, counter-terrorism officials have told the BBC.
The bombings on 7 July last year killed 52 people
Security sources say the situation has never been so grim, said BBC home affairs correspondent Margaret Gilmore.
They believe the network is now operating a cell structure in the UK - like the IRA did - and sees the 7 July bomb attacks "as just the beginning".
Each cell has a leader, a quartermaster dealing with weapons, and volunteers.
According to our correspondent, each cell works on separate, different plots, with masterminds controlling several different cells.
Those involved in the cells were often aware they were being followed and so were meeting in public spaces.
In addition, training is taking place in the UK and Pakistan.
It was thought that five years ago al-Qaeda was a number of "loosely-connected organisations" with common aims, but it is now more organised, she said.
Security officials are concerned the group is targeting universities and the community, and are "less worried" about mosques, she added.
The network is targeting men in their late teens and early 20s, according to our correspondent.
"They set up groups a bit like Boy Scouts or Boys' Brigade... totally legitimate.
"Those who are particularly interested they start giving religious indoctrination.
"Then those who are very interested they start introducing to political teachings, anti-Western rhetoric.
"And those who are still interested they then start giving technical training.
"They also start sending them on bonding sessions to things like white-water rafting.
"You end up with a small team of people - the cell is prepared.
"A lot of this is happening outside London," our correspondent added.
Joint regional offices of MI5 intelligence gatherers and anti-terrorist police officers have been set up in Manchester, Birmingham and Sheffield.
BBC security correspondent Gordon Corera said the view was Britain was particularly vulnerable because "it may be easier for al-Qaeda to strike the UK than other targets".
He said these views were "based on activity they are actually seeing. Plots they're disrupting, trials which might be coming up soon".
"There is hard evidence behind it, rather than just theories," said our correspondent.
"That's based partly on what they are seeing, in terms of the types of activity, and partly based on the coincidence, that al-Qaeda's leadership is based in the tribal areas of Pakistan where there are links to the UK and flows of people going back and forwards.
"It makes it easier to make the UK a target than the other countries it might wish to target."
The network also appeared to be better organised, he continued.
"The leadership of al-Qaeda does appear to have been re-grouping and to be more coherent and organised than had been thought in recent years.
"The view is it clearly was an organised group before 9/11, but the campaign in Afghanistan disrupted that leadership very heavily.
"But in recent years, particularly in the tribal areas between Pakistan and Afghanistan, the al-Qaeda leadership has been able to re-group and re-organise itself.
"In doing so it's able to open up channels of communication, contact, recruitment and planning around the world, and operate those in a more coherent fashion than maybe we were seeing three years' ago."
However, intelligence analyst Crispin Black said another attack in the UK "was not inevitable", citing the UK's "considerable successes against the IRA".
"We still have that expertise and training present within our military forces and intelligence," he said.
"It is no longer about looking for a needle in a haystack. We have some pretty good clues and information on where we should be looking."
A Home Office spokeswoman referred to a recent speech by Home Secretary John Reid in which he referred to the now "seamless threat" of radicalisation.
This was a challenge they expected to "last a generation", she said.