The UK international terror threat level has been raised to "severe".
Home Secretary Theresa May announced that an attack was "highly likely", although there was no evidence to suggest it was "imminent".
Prime Minister David Cameron will make a statement shortly. We'll bring you the latest updates throughout the afternoon.
"Severe" is the second highest of five possible threat levels used in the UK.
Mrs May said the change was related to "developments in Syria and Iraq where terrorist groups are planning attacks against the West".
The home secretary added: "Some of those plots are likely to involve foreign fighters who have travelled there from the UK and Europe to take part in those conflicts."
Nick Robinson, Political editor
tweets: Terror threat raised. Now severe which means attack "highly likely". Remember tho it was that high for many of years since 7/7
Dominic Casciani, Home affairs correspondent, BBC News
tweets: Threat level of "severe" means attack considered "Highly likely". Home Secretary says threat linked to Syria/Iraq rather than a known plot
The UK's threat level is set by the Joint Terrorism Analysis Centre, which is independent of ministers.
You can read more about the Centre here.
Mrs May also called on members of the public to "remain vigilant and to report any suspicious activity to the police".
National Policing Lead for Counter-Terrorism Assistant Commissioner Mark Rowley said visible patrols would be increased from Friday afternoon. He said "other security and protection measures" would also be implemented.
Watch Home Secretary Theresa May's earlier statement here.
The threat level in the UK last changed in July 2011, when it was reduced to "substantial".