Britain raised its terrorism alert yesterday to the second-highest level with Prime Minister David Cameron saying the Islamic State (Isis) group operating in Syria and Iraq posed the country's greatest-ever security risk.
The UK government said there was no evidence an attack was imminent but the assessment of the latest intelligence by security chiefs justified elevating the international threat level to "severe", meaning a strike was "highly likely".
"What we're facing in Iraq now is a greater and deeper threat to our security than we have known before," said Mr Cameron, adding he was "absolutely satisfied that Isis ...would make specific threats to the UK".
It is the first time since mid-2011 that Britain has been placed on this grade of alert by the Joint Terrorism Analysis Centre (JTAC), the independent body responsible for setting the national threat level.
The national threat level was first published in 2006, just over a year after four British Islamists carried out suicide bombings on London's transport network, killing 52 people.
Police chiefs said raising the level to severe would mean a rise in the level of visible patrols, along with other security and protection measures.
White House spokesman John Earnest said: "This is a threat that the United States has been focused on. We've been co-ordinating closely (with Britain and) our other allies in Europe about countering this threat."
Mr Cameron said at a news conference in London yesterday that he will introduce new laws to target terror suspects, pledging to seize passports to fight what he described as an extremist threat more dangerous than any previously seen.
He said the Islamic State group is "effectively a state run by terrorists".
Mr Cameron added: "We could be facing a terrorist state on the shores of the Mediterranean and bordering a NATO member."
The British move comes less than two weeks after a video released by Isis showed the beheading of US journalist James Foley, by a masked militant apparently speaking English with a London accent.