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The UK terror threat level was raised to its highest level of "critical"- meaning an attack may be imminent - after the Parsons Green Tube bombing. It was the second time this year. It has since been lowered back to "severe", meaning an attack is highly likely.

It was last raised to critical following the Manchester Arena bombing in May, which killed 22 people.

The international terror threat scale was first made public in the UK in August 2006, just over a year after the London Tube and bus 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". It has not 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

This is the fourth time the highest level "critical" has been activated.

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 as many as 10 planes flying from Britain to the US.

It was also raised to "critical" following the car attack on Glasgow Airport on 30 June 2007, in which the only fatality was one of the assailants.

And it was put at critical on 23 May this year, the day after the Manchester suicide bombing, and remained there until 27 May, when it lowered to severe.

Northern Ireland scale

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.

How should you respond?

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.

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.

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