The UK terror threat level has been raised to "severe" - meaning an attack is highly likely.
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.
What are the threat levels?
There are five:
- CRITICAL means an attack is expected imminently
- SEVERE means an attack is highly likely
- SUBSTANTIAL means an attack is a strong possibility
- MODERATE means an attack is possible, but not likely
- LOW means an attack is unlikely
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 2017, the day after the Manchester Arena suicide bombing, and remained there until 27 May, when it lowered to "severe".
The threat level was also briefly raised to critical in September 2017 following the Parsons Green attack on the London Underground.
In July 2019, the main terror alert system was changed to include all forms of terror threats "irrespective of the ideology that drives them".
Until this point, it only assessed the threat from "international terrorism" - now it includes right-wing, left-wing and Northern Irish groups.
On 4 November 2019, the threat level was downgraded to "substantial" - the lowest level since August 2014.
It has now been raised again with the Home Secretary Priti Patel calling it "a precautionary measure following the horrific events of the last week in France and last night in Austria and is not based on a specific threat".
Northern Ireland scale
In September 2010, additional scales were introduced to rate the threat level from terrorism related to Northern Ireland.
As part of the changes to the system in 2019, they were reduced to one - for Northern Ireland itself.
Currently, the threat there is assessed as "severe".
Who decides threat levels?
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 is responsible for setting the threat levels from Irish and other domestic terrorism both in Northern Ireland and in Great Britain.
The security service says the threat level decisions take into account a number of factors including:
- Available intelligence - sometimes "specific threat information" but often relying on a wide range of information
- Terrorist capability, irrespective of ideology - including analysis of the potential scale of an attack
- Timescale - the likelihood of an attack in the near term
How should you respond?
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.