In her second statement of the day, Theresa May has announced that the national terror threat level has been raised from "severe" to the highest level,"critical". She added that she has also deployed the military to help armed police by triggering Operation Temperer, a protocol that sends troops to help police with security at airports, rail networks, harbours etc.
This is the first time that the level has been at critical since the foiled 2006 Heathrow plot - defined as the prospect of a terrorist attack moving from being "highly likely" to being "imminent". This has been done before, and both times it lasted for just a few days. But what we haven't seen before is the mass deployment of up to 5,000 armed troops on the streets of Britain. The Prime Minister explained it thus:-
“This means that armed police officers responsible for duties such as guarding key sites will be replaced by members of the armed forces, which will allow the police to significantly increase the number of armed officers on patrol in key locations. You might also see military personnel deployed at certain events, such as concerts and sports matches, helping the police to keep the public safe. In all circumstances, members of the armed forces who are deployed in this way will be under the command of police officers.
The PM emphasised that she does not want the public to feel 'unduly alarmed' but said this was a 'proportionate and sensible response' to the threat. The 'critical' alert may be downgraded soon, but the involvement of the military via Operation Temperer may last for longer. It was deployed before - straight after the terrorist attacks in Brussels last year - but the troops were used covertly, helping out the visible armed police presence in airports and stations.
The decision to increase the threat level is taken by a separate team of security officials inside MI5 and shows the concern at the relative sophistication of last night's attack. And the prospect that a bomb-maker who had passed undetected by MI5 could still be out there, ready to strike again.