The UK's terror threat system explained as level downgraded from critical

The UK's terror threat level has been lowered to 'severe' after several days at 'critical' in the wake of the attack in Manchester.

This means that an imminent terror attack is no longer thought to be on the cards, but that the possibility of an attack remains highly likely.

In August 2014, then Home Secretary, Theresa May announced the terror threat was being raised to severe in the wake of warnings of the deadly threat posed by British jihadists radicalised fighting for Islamic State extremists in Syria and Iraq.

It remained at this level until the attack in Manchester on May 22. Despite terror attacks being highly likely throughout this period there had been no significant attacks on UK soil for more than a decade until the attack at Westminster in March of this year.

Speaking on the BBC's Today programme in the wake of the attack at Parliament, Dominic Grieve, Chair of the Intelligence and Security Committee, said that the security services have foiled 12 attacks in the past 18 months.

However, after the attack in Manchester that has so far left 22 dead and many more injured, Theresa May announced that the terror threat in the UK had been raised to critical, its highest possible level.

Brought in by security chiefs in August 2006, the warning system was part of Britain's new counter-terrorism strategy.

It came a year after the July 7 bombings, in which 52 people were killed and more than 700 injured in coordinated bomb blasts on the Tube and a bus in London.

When they were first published on August 1 2006 the threat stood at severe – the second highest level.

But just over a week later, on August 10, the threat was raised to critical after police exposed a plot to manufacture and smuggle parts of explosives on to passenger jets flying between the UK and the US, and detonate them on board.

Counter terrorism police smashed the terror ring, arresting its members in a raid code-named Operation Overt, and on August 13 the threat level was lowered to severe, where it stayed for around a year.

Britain was plunged into a fresh state of heightened alert and the terror threat was raised to critical on June 30 2007 after a blazing car loaded with propane canisters was driven into a crowded Glasgow Airport.

The attack came just one day after two bombs were discovered in cars in London's Park Lane and Piccadilly – among the capital's busiest tourist spots.

Police quickly arrested Islamist extremist Bilal Abdullah, a man behind the two attempted attacks. And on July 4 2007 the threat level was lowered to severe, where it stood for two years before being lowered again on July 20 2009 to substantial.

But the perils of terrorism came to the forefront again when on Christmas Day 2009 the now-notorious underwear bomber Umar Farouk Abdulmutallab tried to blow up a jumbo jet as it flew to Detroit in the US.

Tragedy was only averted when brave passengers overpowered the Nigerian, and terror chiefs decided to raise the British threat level on January 22 2010 to severe.

It stood at severe for 18 months before being lowered on July 11 2011 to substantial, where it stood for three years.

The level is set by the Joint Terrorism Analysis Centre and the Security Service, which operates out of MI5. It is independent of ministers and assesses a range of factors before deciding on the threat level.