Britain has become the main target of the Al Qaeda terrorist network, which has successfully regrouped and now presents a greater threat than ever before, counter-terrorism officials said.
Intelligence chiefs said the group has substantially recovered its organisation in Pakistan, despite a four-year military campaign to seek out and kill its leaders.
In that time, the organisation has become much more coherent, with a strong core and a regular supply of volunteers, the intelligence chiefs told The Guardian.
More worrying, officials say, is evidence of new techniques that would-be terrorists within the UK have adopted.
The structure of individual Al Qaeda-inspired groups is much more like the old Provisional IRA cells, with self-contained units comprising a lead organiser/planner, a quartermaster in charge of weapons and explosives acquisition and training, and several volunteers.
Officials describe these groups as 'multi-tasking' - involved in fraud and fundraising and courier work as well as planning attacks.
"There is a hierarchy within each cell with a very tightly-run command and control," said one source.
Many suspects appear to be aware they are under surveillance and have taken to having important conversations outside - in parks and other public spaces - similar to the tactics used by PIRA leaders during the Troubles.
Intelligence experts fear the UK is a target as never before, with extremists intent on carrying out a huge spectacular, on the scale of the US atrocities in 2001.