Critical threat level: What does it mean, and what is Operation Temperer?

Prime Minister Theresa May has raised the terror threat in the UK to its highest level for the first time in a decade and activated Operation Temperer - allowing military personnel on the streets to free up police resources - for the very first time.

The announcement follows a blast outside a concert in Manchester which left 22 people dead.

After 22-year-old Salman Abedi was identified as the suspected suicide bomber, the threat level was eventually raised to critical and the Operation Temperer was established.

What is a critical threat level?

Critical threat levels means that a terrorist attack is expected imminently. Ms May said that the decision to raise the threat level to critical was taken in part because of the potential that the suspected Manchester attacker may have been part of a network of other terrorists who may attempt another attack.

What is Operation Temperer?

Operation Temperer is an established security measure that means citizens can expect to see soldiers helping law enforcement at sensitive security points like rail stations, Parliament, and other destinations or events with big crowds. The operation, first unveiled in 2015, is a practised procedure, and allows UK soldiers to replace armed police in many areas so that those police can patrols in key areas.

The plan will reportedly allow as many as 5,000 troops to be deployed in the UK.

But military assets have been used to protect against the terrorist threat before, including in February 2003 when troops were drafted in to Heathrow, with armoured vehicles patrolling amid fears that extremists could be on the loose with a surface-to-air missile launcher.

When has the threat level been raised to critical before?

It has been a decade since the threat level was last raised to its highest point. On 10 August 2006 — a little over a week after the first publication of a terror threat level — the level was raised from severe to critical when a plot to blow up as many as six to 10 flights leaving London Heathrow airport was thwarted.

The second time came on June 30, 2007, when a dark green Jeep Cherokee loaded with propane canisters was driven into Glasgow Airport and set on fire. That jeep was unable to enter the terminal, but several people including the driver were severely harmed. There were no deaths.

Two car bombs had been discovered just a day earlier in London. They were disabled before they could be detonated.

How long have critical threat levels lasted before?

The instance in 2006 lasted just three days and the incident in 2007 lasted four days.