After the tube bombing attack in London on Friday the terrorism threat level reached the highest level of ‘critical’.

The terror threat level has now been lowered again to ‘severe’, Home Secretary Amber Rudd confirmed on Sunday.

This means an attack is highly likely, and follows the terror threat level temporarily being raised to critical on Friday, meaning an attack may be imminent.

In Hull, we have seen more armed police at major events and in places like St Stephen's shopping centre.

Armed police chat to shoppers in St Stephen's
Armed police chat to shoppers in St Stephen's

But what do these terrorism threat levels actually mean and what do we need to do? Here we provide some answers.

What are terror threat levels and why do they exist?

The UK national threat level system was introduced by the Blair government in 2003.

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 tend to attract media attention.

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

Ferry terminal at King George Dock, east Hull
Ferry terminal at King George Dock, east Hull

There are five levels of threat:

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

Why was the threat level at critical?

Friday’s decision to raise the UK threat level to critical and then reduce it again to severe, 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.

The precise details of the intelligence and information JTAC use to reach their decision to change the threat level are not made public.

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 reached critical before?

The threat level has only been at critical three times. It reached critical in August 2006 after a plot to blow up planes from the UK to the US was uncovered. It reached that level again in June 2007 after the attempted bombing of Glasgow airport.

CRITICAL: The Prime Minister has raised the terror threat level to 'critical' after the explosion in Manchester

In May this year it reached critical once more following the Manchester Arena bombing.

On August 29, 2014 the UK threat level from international terrorism was changed from substantial to severe and remained at this level until May this year.

What have Humberside Police said?

Over the weekend, Humberside Police stepped up its armed police patrols but encouraged members of the public not to feel intimidated.

Assistant Chief Constable Steve Graham said: "We would like to take the opportunity to thank members of our communities, many of whom have expressed huge support and thanks to our officers both in person and on social media over the weekend.

"Your support is greatly appreciated by us all and really makes a difference. We’d also like to thank our partner organisations for their swift and thorough actions in response to the increase in UK security levels."