The UK has five security threat levels which are designed to give a broad indication of the likelihood of a terrorist attack.
They are Low (an attack is unlikely); Moderate (an attack is possible, but not likely); Substantial (an attack is a strong possibility); Severe (an attack is highly likely); Critical (an attack is expected imminently).
Threat levels are set by the Joint Terrorism Analysis Centre, a group of police and security service counter-terror officials set up in 2003.
JTAC bases its decisions on a range of factors including intelligence, recent events and what is known about terrorist intentions capabilities.
The raising of the threat level from Severe to Critical, as happened yesterday, automatically triggers maximum protective measures.
Such a move would be a response to intelligence about specific, imminent threats, according to MI5's website.