The terror threat level in the UK has been raised from "substantial" to "severe", meaning an attack is "highly likely".
Home Secretary Alan Johnson has sought to reassure the public, saying: "I have to emphasise the fact it does not mean an attack is imminent."
He said the Joint Terrorism Analysis Centre keeps the threat level under constant review and makes its judgements based on a "broad range of factors".
Mr Johnson said those factors include "the intent and capabilities of international terrorist groups in the UK and overseas".
He went on: "We still face a real and serious threat to the UK from international terrorism.
Terror Threat LevelsCritical - attack is expected imminently
Severe - attack is highly likely
Substantial - attack is a strong possibility
Moderate - attack is possible, but not likely
Low - attack is unlikelySKY News
