Q&A: Just how severe is UK terror threat?

A serviceman was threatened with a knife near RAF Marham in Norfolk

• How severe is severe?

The severe rating indicates that an attack is seen as highly likely.

• How many levels are there?

Severe is the second highest out of five, behind critical which means an attack is expected imminently. The other three levels are substantial (an attack is a strong possibility), moderate (an attack is possible but not likely), and low (an attack is unlikely).

• Who decides the threat level?

An organisation called the Joint Terrorism Analysis Centre. It was established in June 2003 and is based at MI5’s London headquarters. JTAC is comprised of representatives from 16 government departments and agencies.

• How long has the threat level been at severe?

Almost two years. It was raised from substantial in August 2014 after the rise of Islamic State.

• How has the threat changed over the years?

The level was first made public in 2006. Since then, it has most often been at the severe level, and not been lower than substantial. The critical level has been put in place twice in the past decade - August 2006 and June 2007. On both occasions the assessment was lowered after a few days.