Britain has raised its terror threat level from 'substantial' to 'severe' - its second highest level.
Home Secretary Alan Johnson says the change indicates a terrorist attack is highly likely, though there's no intelligence to suggest that an attack is imminent.
The decision to raise the threat level was made by the UK's Joint Terrorism Analysis Centre.
Mr Johnson said decision was not specifically linked to the failed Christmas Day bomb attack on a plane bound for Detroit, or to any other incident.
The US Department of Homeland Security said the move meant Britain would be on a similar level of alert to America.
The change in threat level comes days ahead of two major international conferences on Yemen and Afghanistan in London on Wednesday and Thursday.
The UK had not reached the highest threat level of "critical", which would mean an attack was imminent.