The international threat to the UK from a terrorist attack has been downgraded from 'severe' to 'substantial', Home Secretary Theresa May said on Monday.
The threat level was last altered in January 2010, when it was increased to "severe".
The new threat level means that while an attack is not "highly likely" it is still "a strong possibility" and could happen "without further warning", May said.
In a statement May said: "The change in the threat level to substantial does not mean the overall threat has gone away - there remains a real and serious threat against the United Kingdom and I would ask the public to remain vigilant."
She added that the decision to change the threat level "is taken by the Joint Terrorism Analysis Centre independently of ministers and is based on the very latest intelligence, considering factors such as capability, intent and time-scale."
The terrorism threat level was first made public on August 1 2006, and was raised to critical ten days later after arrests were made connected to a plot to blow up transatlantic flights.