Home Secretary Theresa May first announced the increased terror threat level on Friday, followed swiftly by a prime ministerial press conference on Downing Street. The "severe" perceived threat of an attack on UK soil classifies the probability as "highly likely," but does not indicate that authorities consider an attack "imminent."

"The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West," the minister for the interior, May, said. "Some of those plots are likely to involve foreign fighters who have traveled there from the UK and Europe to take part in those conflicts."

Prime Minister David Cameron subsequently said that at least 500 people had left the UK "to fight in Syria and potentially Iraq." One of them is believed to be the man who beheaded US journalist James Foley in a video published by the self-proclaimed "Islamic State" (IS) terror militia.

The "IS" has seized control of territory in both Iraq and Syria, prompting a string of international sanctions and US airstrikes against some targets in Iraq. Several European countries, including the UK and Germany, are in the process of arranging weapons deliveries to bolster the supplies of Kurdish peshmerga forces in Iraq fighting IS' push towards the Turkish border.

Cameron said that the change in the threat level would not impact most Britons' daily lives, encouraging people to go about their business as usual. He said that the change would mainly impact on domestic security measures, for instance the proportion of armed police - still the exception rather than the rule in the UK - deployed in certain locations.

Both May and Cameron stressed that the decision was made by the country's Joint Terrorism Analysis Centre, not the government. Britain's threat level, made a matter of public record in 2006, had stayed static for more than three years. It was most recently rolled back one notch from "severe" to "substantial" in July 2011 - a change reversed on Friday.

msh/jr (AFP, AP, dpa)