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This is the fourth time the UK national terror threat level has been raised to 'critical' since the system was made public in 2006.

The last time was in May this year, following the Manchester Arena bombing, when it was wrongly thought that the bomb-maker was still at large and could strike again.

In the case of Parsons Green it is perhaps surprising that it took the government so long - more than 12 hours - to raise the threat level to critical, under advice from the Joint Terrorism Analysis Centre, when it was obvious the perpetrator had neither died nor been caught.

Each time the level has gone to critical, it has only stayed at this highest level of alertness for three to four days - this is partly as it involves an unsustainably high tempo for the police, intelligence and security services.

Read the full analysis here.