Paris attacks: What are the five levels of UK terror threat?

A police officer at Gatwick Airport's North Terminal, which was  cordoned off today as a 41-year-old man from France was questioned after "what appears to be a firearm" was discovered at the airport,
A police officer at Gatwick Airport's North Terminal, which was cordoned off today as a 41-year-old man from France was questioned after "what appears to be a firearm" was discovered at the airport,

The UK's official terror threat level will come under scrutiny in the wake of the Paris attacks.

The assessment is made by the Joint Terrorism Analysis Centre (JTAC) and has been made public since 2006.

It has mainly stood at either of the top two levels - critical or severe - since it was first published.

Here are the different categories:

1. Critical

This is the most severe level and indicates that an attack is expected "imminently". It has been put in place on two occasions since the information was first published, in August 2006 and June 2007. On both occasions the level was lowered after a few days.

2. Severe

This means security services see an attack as "highly likely" and is the category currently in place. The threat has been assessed as severe since it was raised in August last year following the rise of Islamic State.

3. Substantial

This indicates that an attack is judged to be a "strong possibility". The level has been at substantial in July 2009 and then between July 2011 and August 2014.

4. Moderate

This means an attack is "possible, but not likely". The level has not been at moderate since it was first published.

5. Low

This means an attack is "unlikely". The threat assessment has not been at this level since it was first published.