Reality Check: How terrorism threat levels work
In the wake of the Manchester Arena attack on Monday, the UK terror threat level was raised to its highest level of "critical", amid fears that more attacks may be imminent.
Home Secretary Amber Rudd told BBC Radio 4's Today programme that this meant the police would have 3,800 soldiers to call on and equipment for "as long as we need them".
The international terror threat scale was first made public in the UK in August 2006, just over a year after the London bombings that killed 52 people.
There are five ratings at which the threat level can stand. Since its introduction, the level has never fallen below what is known as "substantial", and it hasn't been that low since August 2014.
MI5 Threat level descriptions:
- LOW means an attack is unlikely
- MODERATE means an attack is possible, but not likely
- SUBSTANTIAL means an attack is a strong possibility
- SEVERE means an attack is highly likely
- CRITICAL means an attack is expected imminently
The highest level "critical" has been activated twice before.
The threat level was first raised to "critical" on 10 August 2006, just nine days after the scale was published, following the discovery of a plot to blow up planes flying from Britain to the USA on as many as 10 flights.
The only other time the threat level was raised to "critical" was following the car attack on Glasgow Airport on 30 June 2007, in which the only fatality was one of the assailants.
On neither occasion was the threat maintained at that highest level for longer than four days.
Before being moved up to "critical" on 23 May 2017, the threat level had been held at "severe" for 998 days.
In September 2010, additional scales were introduced to rate the threat level from terrorism related to Northern Ireland.
There is one scale for potential action within Northern Ireland and another for terrorist action on the Great Britain mainland. Currently those levels for the risk from terrorism associated with Northern Ireland, are assessed as "severe" within Northern Ireland and "substantial" for the GB mainland.
The threat level for the UK from international terrorism is set by the Joint Terrorism Analysis Centre (JTAC), which is a self-standing organisation made up of representatives from 16 government departments and agencies.
How should you respond?
MI5 says the scale is intended to be a tool for security practitioners and the police to determine what security response may be required.
Threat levels in themselves do not require specific responses from the public.
The MI5 website says: "Vigilance is vital regardless of the current national threat level," and that "sharing national threat levels with the general public keeps everyone informed."
It adds that making the threat level public "also helps explain the context for the various security measures (for example airport security or bag searches) which we may encounter in our daily lives."
The website urges members of the public with information about possible terrorist activity to call the Anti-Terrorist Hotline on 0800 789 321.