How has Parsons Green blast affected UK's official terror threat level?

The official terror threat level for the UK remains at severe after the Parsons Green blast, but the assessment is being kept under review.

What does the severe rating mean?

That authorities believe an attack is "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?

A body 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 is a judgment reached on the threat level?

A number of factors may be taken into account, including available intelligence, terrorist capability and terrorist intentions.

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 in place three times over the 11-year- period -in August 2006, June 2007 and in May this year after the Manchester bombing. On all three occasions the assessment was lowered after a few days.