Theresa May announced on Tuesday that the UK’s terror threat level has now risen to critical, following the attack on a Manchester music concert in which 22 people are now known to have died.

Since then, nearly a thousand troops have been deployed to UK streets and high-profile public areas, including outside Buckingham Palace and Downing Street.

What are terror threat levels and why do they exist?

The UK national threat level system was introduced by the Blair government in 2003, following a recommendation from the parliamentary Intelligence and Security Committee the previous year.

The idea is to make sure that anyone responsible for keeping critical national infrastructure – such as transport, water, gas and electricity networks – safe from attack takes appropriate steps.

Changes in the threat level are not specifically intended to inform the wider public, but they naturally attract media attention, as they did this week.

What do the different levels mean and how are they set?

There are five levels of threat:

  • Low, an attack is unlikely
  • Moderate, there is a possibility of an attack but it’s unlikely
  • Substantial, an attack is a “strong possibility”
  • Severe, an attack is “highly likely”
  • Critical, an attack is “expected imminently”

Tuesday’s decision to raise the UK threat level to critical was taken by the Joint Terrorism Analysis Centre. JTAC is an agency of MI5 that pulls together information from across the intelligence and security services to assess the level of threat the UK faces from terrorism.

MI5 says that JTAC judges the threat based on several factors. These include the available intelligence, what is known about the capabilities of terrorists, terrorist intentions, and when an attack might happen.

Why is the threat level now at critical?

The precise details of the intelligence and information JTAC have used to reach their decision to raise the threat level to critical have not been made public. But we’re told that the decision reflects an estimation by the security services that another attack could be imminent. Troops have been deployed and counter-terror operations are ongoing, with eight men currently in custody.

Why are there troops on the streets and has it happened before?

Troops have been deployed to key locations across the UK as part of Operation Temperer, in order to support armed police. This is the first time it has been triggered since plans were drawn up (and accidentally published) in 2015, after the Charlie Hebdo attacks in Paris that year.

The decision to enact Operation Temperer was not directly triggered by the JTAC’s announcement to raise the threat level, although clearly the two are linked. According to the Prime Minister, following JTAC’s revised assessment of the terror threat, the police made a request for troops to be deployed, which was approved by the Secretary of State for Defence, Michael Fallon.

This is not the first time that troops have been deployed on UK streets. Operation Banner saw British armed forces patrol locations in Northern Ireland between 1969 and 2007 – the longest continuous deployment in British history.

Tony Blair sent tanks and 450 military personnel to Heathrow airport in 2003 after a plot was uncovered to bring down a passenger jet with a surface-to-air missile. That was the first deployment of troops in mainland Britain since the police strikes of 1919.

Should I do anything?

MI5 say that “threat levels in themselves do not require specific responses from the public”, but that “vigilance is vital regardless of the current national threat level”.

They encourage anyone with information about possible terrorist activity to call the Anti-Terrorist Hotline on 0800 789 321.

Has the UK threat level ever been this high before?

The last time the threat level was at critical was in 2007, after the attempted bombing of Glasgow airport. There were no further attacks following that incident, although it is thought to be linked to a car bomb that was disabled before it could detonate in central London the previous day. The threat level was downgraded to severe five days later.

How do other countries measure terror threats?

USA: In the US, the Department of Homeland Security operates the National Terrorism Advisory System (NTAS), which issues public alerts when it has “information about a specific, credible threat”. Unlike the UK’s system, NTAS is specifically designed to give the public practical tips and to share details where possible of the type of threat that exists.

NTAS has three categories of severity: elevated, intermediate and imminent. It replaced the colour-coded ranking of terror threat introduced by George Bush after 9/11, which was criticised by political and satirical commentators for the administration’s perceived use of the system to justify counter-terror crackdowns.

France:  The French Vigipirate system, also known as Plan Vigipirate, was created in 1978 by then-President Valéry Giscard d’Estaing. It has two levels of severity: “vigilance” and “attack alert”. Like the American NTAS, the Vigipirate system also instructs the public on how they should respond to the threat level.

France has been in a state of emergency since the November 2015 Paris attacks in which 130 people died. President Emmanuel Macron, who took office last month, will ask the French parliament to extend the state of emergency until 1 November in response to this week’s Manchester bombing.