London:
British Home Secretary Theresa May has said the country's terror threat risk level was being raised to "severe" due to fears over the situation in Iraq and Syria.
"The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West," May said.
"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."
The "severe" threat level means an attack is "highly likely", although May added there was no intelligence to suggest that an attack was imminent.
The threat level is now at the second highest level out of five possible categories and is at its highest since July 2011.
Prime Minister David Cameron was due to make a statement on the raising of the alert level at 2.45 pm (1345 GMT).