​The UK's terrorism threat level has today been upgraded from 'substantial' to 'severe.'

The move means security chiefs believe that an attack is highly likely, but there is no specific intelligence of an imminent incident. During this time, people are encouraged to be alert, but not alarmed.

The assessment of threat levels are taken by the Joint Terrorism Analysis Centre (JTAC), which makes its recommendations independently from the government. The five levels of threat set by the JTAC are as follows:

• Low - an attack is highly unlikely
• Moderate - an attack is possible but not likely
• Substantial - an attack is likely
• Severe - an attack is highly likely
• Critical - an attack is highly likely in the near future

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.

The UK's terrorism threat level was raised to the highest rating, 'critical', in the days following the Manchester Arena bombing in May 2017.​

It last reached that level again briefly in September that year, after a bomb partially exploded on a Tube train at Parsons Green.

The threat level remained at the second highest rating until last November when it was downgraded to 'substantial,' where it has stayed until this point.

SecuriGroup's Senior Operations Director, Allan Burnett QPM (former Counter Terrorism Coordinator for Scotland and current Chair of the Scottish Business Resilience Centre's SISS-CT Group) commented:

'The increase in the threat level to SEVERE, meaning an attack is highly likely, requires all of us to ensure we are at our most vigilant. Let's do everything we can in support of the police and our security services to keep people safe.

If you have information about possible terrorist activity, call the Anti-Terrorist Hotline: 0800 789 321. The Anti-Terrorist Hotline is for tip-offs and confidential information. For warnings about possible bombs or other urgent threats, call 999.'