It is only the third time in the history of the UK's terrorism warning system that the terror threat has been raised to critical.

Following the Manchester Arena bombing which killed 22 people, including children, and injured a further 64, Prime Minister Theresa May announced the threat level change on Tuesday night.

She said the decision had been made due to the possibility there are "a wider group of individuals linked" to the attack.

As a result, Operation Temperer has been put into effect, which has seen members of the armed forces deployed in certain areas to bolster police.

But how is the terror threat level calculated - and what does it mean for you?

Raising the threat level to critical means another terror attack is considered to be imminent.

Brought in a year after the 7/7 London Tube bombings which killed 56 people in 2005, the system has five gradings - low, moderate, substantial, severe and critical, which is the highest possible threat level.

For the majority of the 11 years since the system's implementation, the threat level has been at severe, which means an attack is highly likely.

Until Tuesday's announcement, the threat level had been at severe since August 2014, when May, then home secretary, announced it had been changed from substantial due to the rise of so-called Islamic State in Iraq and Syria.

Charlie Bird, former diplomat and a terrorism expert based at St Andrews University, said: "The fact that this is the first time it has been raised to critical since 2007 suggests there is credible intelligence of another attack being imminent.

"That could be because the intelligence and security agencies have discovered intelligence through their investigation into Monday night's bombing that there are other bombs out there and other people out there.

"Or there could be intelligence that has been gathered by one of our intelligence allies and provided to UK authorities."

The UK's threat level from international terrorism is worked out by the Joint Terrorism Analysis Centre (JTAC).

Based at MI5 headquarters in Thames House, London, JTAC brings together counter-terrorism experts from the police, intelligence agencies and government departments to regularly review the threat level.

They examine available intelligence and other information relating to terrorist capability and intentions, and analyse the likelihood of terror attacks in the near term.

Before Tuesday's announcement, the threat level has only ever been raised to critical on two occasions.

Only ten days after the system was implemented, on August 10, 2006, a critical threat level was announced after police uncovered a plot to bomb as many as ten passenger jets flying from the UK to the US.

The threat was again raised to critical by JTAC on June 30, 2007 - the day of an attempted terror attack on Glasgow Airport when saw a blazing car filled with propane canisters rammed into the terminal.

The Glasgow attack came only a day after two car bombs were discovered and disabled in Park Lane and Piccadilly in London.

Allan Burnett, former counter-terror police chief for Scotland, said: "In recent times we have seen what I would describe as reasonably unsophisticated attacks, both in Europe and in the United Kingdom.

"Any experience we have had when there has been an improvised explosive device is that a much greater degree of planning and sophistication has to go into that.

"That would point to a group of people rather than an individual."

Historically, JTAC has only kept the threat level at critical for a few days.

In 2006, police and security services smashed the terror ring plotting the transatlantic airliner attacks and the threat level was reduced to severe on August 13.

Iraqi doctor Bilal Abdullah was arrested at Glasgow Airport on June 30, 2007, for his role in both the Glasgow and London terror plots. He was later sentenced to life in prison.

His accomplice Kafeel Ahmed, who drove the explosive-laden Jeep into the Glasgow Airport terminal, died in hospital from burns four weeks after the attack.

The threat level was announced as lowered back to severe by then-home secretary Jacqui Smith on July 4, 2007.

"You can really only sustain it for a limited amount of time but in that time an awful lot of work can be done," said Mr Burnett.

"There is the operation part, which you would hope can be brought to a speedy and safe conclusion, but there is also the reassurance part."

Operation Temperer is a UK Government plan in which up to 5000 military personnel can be deployed in key areas in the event of a major terrorist incident.

They are most likely to be deployed at places like major transport hubs, government buildings and large events where they can offer assistance to police officers.

The last time the plan was put into effect was immediately after the Brussels attack in March last year, although military personnel only supported police officers covertly as back-up.

All troops on the streets will be working under the command of police officers.

Mr Burnett explained: "From the point of view of the military aid that could be brought in, what they offer is range of specialisms - they can be armed, to state the very obvious, and they also bring in search expertise.

"It can also free up police resources to go about their very important duties in catching any remaining people in the gang."

People in Scotland have been urged to remain "vigilant but not alarmed" by First Minister Nicola Sturgeon.

UK armed forces are guarding 12 sites in Scotland - nine Ministry of Defence sites and the three civil nuclear sites at Dounreay, Hunterston and Torness.

This has freed up armed police officers who usually patrol these sites to be part of a contingency force Police Scotland can call upon.

Armed police have been deployed at several locations across Scotland, including at transport hubs such as train stations.

The First Minister has urged Scots to go about their business as normal as far as possible and said "no specific threat to Scotland has been identified".

She added that the Scottish Government does not "currently envisage" members of the armed forces being deployed on Scotland's streets.

Police Scotland's chief constable Phil Gormley said: "We are reviewing all significant events along with event organisers taking place within the next 14 days and will increase the security footprint around those eve nts where it is deemed appropriate."

One such event will be Saturday's Scottish Cup Final at Hampden between Aberdeen and Celtic.

It has already been announced there will be additional safety measures at the match including full body and bag searches.

Police have said "appropriate" resources will be deployed for former US president Barack Obama's visit to Edinburgh on Friday.

The Edinburgh Marathon also take place this weekend, with around 7000 runners expected to participate along with thousands of spectators.

Global security expert Professor Peter Jackson, based at Glasgow University, said: "I don't know whether it is a matter of time - I don't know whether Scotland is a particular target.

"But there is no question that Scotland is not entirely secure. No place in all of Europe is secure from this kind of attack.

"They are almost impossible to prevent entirely."