Britain has lowered its assessment of a terrorist attack from "severe" to "substantial," Home Secretary Theresa May said Monday.
The new alert level meant that the risk of an attack is a "strong possibility" and may occur "without further warning."
May did not explain the reasons that led to the change, saying only that it was based on the "very latest intelligence" and considering factors including capability, intent and timescale.
There are five levels of threat, from low, meaning an attack is unlikely, to critical, meaning an attack is imminent. "Substantial" is the third level.
The threat level was last at critical in June 2007, following an attack on Glasgow Airport and failed car bombings in central London.