By Gordon Corera
Security correspondent, BBC News
The Joint Terrorism Analysis Centre decides the threat level
Days after an alleged bomb plot was said to have been disrupted, the terror threat to the UK was downgraded from "critical" to "severe". Why?
The decision to lower the threat level was taken late on Sunday night at a meeting of JTAC - the Joint Terrorism Analysis Centre which is housed within MI5.
JTAC comprises of officials from across all of Britain's intelligence agencies and government departments.
It had been meeting regularly a number of times each day since the current crisis began, to assess all the available intelligence.
Officials are emphasising that even with the lowering of the threat level, the current state, severe, indicates that they believe an attack is still highly likely. They simply no longer believe it is imminent.
Police and security services have been trying to establish whether there could be any further associates linked to the alleged plot
Over the last few days, the police and security services have been trying to establish whether there could be any further associates linked to the alleged plot who remain at large and who are capable of still carrying out some kind of attack.
The home secretary had emphasised that the "known" players had been picked up but that still left possible "unknown" players.
Sources are still not ruling out that this could be the case and that others could still be out there but say that for now the intelligence does not lead them to see an attack as imminent.
7 July experience
They also remain worried about groups who had no direct connection to this latest alleged plot, but who might be inspired by its thwarting to carry out their own attack in order to prove that the capacity to do harm still exists.
This concern is based on the experience after the 7 July bomb attacks in London last year, when a number of plots were foiled and some of them were thought to have been brought forward either because of 7 July or the failure of the attempted bombings on 21 July.
Moving the threat level can be politically sensitive
Overall, sources have told the BBC that they are currently watching "dozens" of plots at varying stages and that it would be "very na´ve" to think that the current threat has passed.
Ministers are not involved in the decision-making process for lowering the threat level and JTAC is supposed to be insulated from external pressures and considerations.
In other words, it is supposed to only look at the available intelligence on terrorist threats and not take into account factors like public opinion, airport chaos, or the economic impact of a particular threat level.
There is a sharp distinction between the threat assessment which is a professional judgement of JTAC and what actions are taken by the government based on that assessment.
The latter is the domain of ministers and officials, often acting through the Security and Intelligence Co-ordinator Sir Richard Mottram and through meetings at COBR - the Cabinet Office Briefing Room.
Moving the threat level can be politically sensitive though. It was lowered just before the 7 July attacks last year and was due to be lowered around the time of the 21 July attempted bombings.
Since its movements were being leaked and were considered confusing, the government decided to institute a new, more public, and simpler system.
The new system was only introduced on 1 August. Few could have expected it would have seen so much activity so early on.