The terror threat in Britain has been raised from "substantial" to "severe" in response to conflicts in Iraq and Syria, meaning that an attack is "highly likely".
There is no intelligence to suggest an attack is imminent, the Home Office stressed, but it is the highest level of alert since 2010, when the Joint Terrorism Analysis Centre (JTAC) took the decision on a "broad range of factors".
It is the fourth-highest out of five levels and only twice - in August 2006 and June 2007 - has it been raised to "critical".
The move in 2007 came after the attack at Glasgow International Airport, when a burning Jeep full of gas cannisters was driven into the terminal, and a failed car bomb attack in London the previous day.
In 2006, the change followed the discovery of a foiled plot to blow up to 10 planes flying from the UK to the US using liquid explosives smuggled inside hand luggage.
According to the MI5 website, threat levels are designed to give a "broad indication" of the probability of a terrorist attack.
The threat for Northern Ireland is calculated separately to England, Wales and Scotland and is currently also "severe", unchanged from 2010.
The levels are based on the assessment of a range of factors including current intelligence, recent events and what is known about terrorist intentions and capabilities.
"This information may well be incomplete and decisions about the appropriate security response are made with this in mind," the site says.
Security services and police change their behaviour accordingly and assessments are produced when deemed necessary for public figures and events.
Definitions of UK terror threat levels:
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 imminentlyReuse content