The UK terror threat level is being raised from "substantial" to "severe", Home Secretary Alan Johnson has said.
The new alert level means a terrorist attack is considered "highly likely". It had stood at substantial since July. It is in response to the perceived increased threat from international terrorism following the failed Detroit airliner bombing on Christmas Day.
Mr Johnson stressed there was no intelligence to suggest a terrorist attack was imminent.
The decision to raise the threat level was made by the UK's Joint Terrorism Analysis Centre (JTAC).
 | THREAT LEVEL SYSTEM Low - attack is unlikely Moderate - attack is possible, but not likely Substantial - attack is a strong possibility Severe - attack is highly likely Critical - attack is expected imminently  |
He said: "We still face a real and serious threat to the UK from international terrorism, so I would urge the public to remain vigilant and carry on reporting suspicious events to the appropriate authorities and to support the police and security services in their continuing efforts to discover, track and disrupt terrorist activity."
Mr Johnson said the new level meant people needed to be "more aware".
The decision to raise the threat level was not specifically linked to the failed Christmas Day bomb attack on a plane bound for Detroit or to any other incident, he said.
He said the government would not reveal specific intelligence details.
"We never say what the intelligence is and it would be pretty daft of us to do that," he said, adding: "It shouldn't be thought to be linked to Detroit or anywhere else for that matter."
But the UK had not reached the highest threat level of "critical", which would mean an attack was imminent.
Move mirrors US
He added: "We have a very adept and very focused counter terrorism facility in this country, which consists of many police officers as well as security officers, so the public should be reassured by that."
The US Department of Homeland Security said the move meant the UK would be on a similar level of alert to America.
 The threat level was last at severe in July 2009 |
"We have enhanced our security measures and communicated specific information to industry, law enforcement and the American people."
BBC home affairs correspondent Daniel Sandford said the perceived threat from Yemen since the Christmas day attempted attack may be one factor behind the decision to raise the threat level.
But he added that there might be additional factors which have not been revealed by the government.
Prime Minister Gordon Brown announced on Wednesday that direct flights between Yemen and the UK were to be suspended over fears about their safety.
The change in threat level comes days ahead of two major international conferences on Yemen and Afghanistan in London on Wednesday and Thursday.
There are five levels of terror threat, ranging from low - meaning an attack is unlikely - to critical - when an attack is expected imminently. Severe is the second highest level on the scale.
The threat level was first made public on 1 August 2006, when it was set at severe. It was raised to critical on 10 August 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.
