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
Mr Johnson said JTAC kept the threat level under constant review, making its judgments based on a broad range of factors, including the intent and capabilities of international terrorist groups in the UK and overseas.
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.
The UK had not reached the highest threat level of "critical", which would mean an attack was imminent.
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."
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.