The UK's "critical" status is one of five security threat levels.
The threat levels are:
Low - meaning 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
The level is set by the Joint Terrorism Analysis Centre (JTAC), which UK authorities created in June 2003.
The last time the level was raised to critical was last August, after police said they had foiled a plot to blow up flights between UK and the US.