Threat levels are designed to give a broad indication of the likelihood of a terrorist attack.
The threat level from international terrorism is set by JTAC - the Joint Terrorism Analysis Centre.
It sits within MI5 and is deliberately independent of government. JTAC constantly reviews the threat level, daily.
PM: A further terror attack may be imminent
Their decision to raise or lower it is based on objective analysis of current intelligence, not as a result of the political desires or motivations of the government of the day.
The main UK threat level has been at either Severe (meaning an attack is "highly likely") or Critical (an attack is "imminent") since 29 August 2014.
What to do in a terror attack
Intelligence assessments are always best judgements based on available information - they are rarely exact.
The five threat levels are as follows:
:: LOW means an attack is unlikely
:: MODERATE means an attack is possible, but not likely
:: SUBSTANTIAL means an attack is a strong possibility
:: SEVERE means an attack is highly likely
:: CRITICAL means an attack is expected imminently