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.
Britain is an easier target, they have concluded, because of its traditional links with Pakistan, which is visited by thousands of people each year.