The international terror threat level to the UK has been reduced from severe to substantial, Home Secretary Theresa May said today.
But a terrorist attack remains a strong possibility and might well occur without further warning, she said.
Mrs 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."
Mrs May went on: "Today the Joint Terrorism Analysis Centre (JTAC) changed the UK threat level from international terrorism from severe to substantial.
"This means that a terrorist attack is a strong possibility and might well occur without further warning.
"The decision to change the threat level is taken by JTAC independently of ministers and is based on the very latest intelligence, considering factors such as capability, intent and timescale.
"Substantial continues to indicate a high level of threat and the threat level is kept under constant review."
There are five levels of threat, ranging from low, meaning an attack is unlikely, to critical, when an attack is expected imminently.
The threat level was first made public on August 1, 2006, when it was set at severe. It was raised to critical on August 10 that year after a series of arrests over an alleged plot to blow up transatlantic aircraft, but lowered to severe again the following week.
The threat level was last at critical in June 2007, following the attack on Glasgow Airport and the failed car bombings in central London.
The level is under constant review and can change quickly in response to events.
The threat from dissident Republican and Loyalist terrorists in Northern Ireland was increased to substantial in September last year, meaning an attack was a "strong possibility".
It followed a warning from the head of the MI5 that dissident Irish republicans could attempt to mount a wave of terrorist attacks on the British mainland.
Mrs May urged people to remain vigilant for terrorist threats following the death of Osama bin Laden in May.
The world's most wanted man, who was held responsible for the September 11 terror attacks in New York in 2001 and the London bombings of July 7 2005, was killed in a US operation in Pakistan.
Speaking days after bin Laden's death, Scotland Yard chief Sir Paul Stephenson also urged communities to remain vigilant.
Sir Paul said: "Osama bin Laden led an organisation which is responsible for the injury and death of thousands of people worldwide in the name of an extreme and perverted ideology, committed to the use of terror and murder to achieve their aims.
"However, one man's death does not mark the end of an ideology and we must remain alert to the continuing threat from al Qaida, its affiliates and those acting alone."
But the Metropolitan Police Commissioner went on: "We cannot do this alone, we need the help of the public to protect the country from the threat of terrorism."