Britain raised its terrorism alert on Friday to the second-highest level, with Prime Minister David Cameron saying the Islamic State (IS, formerly ISIS) group operating in Syria and Iraq posed the country's greatest ever security risk.
According to a report by the Reuters news agency, the government in London 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 with IS is a greater and deeper threat to our security than we have known before," said Cameron, adding he was "absolutely satisfied that IS ... 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, according to Reuters.
The British move comes less than two weeks after a video released by IS showed the beheading of U.S. journalist James Foley, by a masked knifeman apparently speaking English with a London accent. An investigation to identify the suspected attacker is in train.
British and European authorities have been warning for many months the Syrian conflict posed a serious terrorism threat, but no specific reason was given why JTAC had raised its alert.
"The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West," Home Secretary Theresa May said in a statement quoted by Reuters.
"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," she added.
The high alert comes just hours after two British IS fighters currently in Syria, Abu Bakr and Abu Anwar, were interviewed via a video feed by CNN and one of them hinted he would be ready to come back to Britain.
The UK has been on alert for the danger of jihadists returning from Syria for a while.
Last October, MI5 Chief Andrew Parker warned "that there are several thousand Islamist extremists here who see the British public as a legitimate target." That threat was illustrated earlier last May, when a 25-year-old British soldier was brutally beheaded by Muslims in the middle of a London street.
Indicating the level of the British threat, the CIA sent a special contingent of agents in June to investigate the extent of Islamic extremism in the UK, in a snub to British intelligence agencies.
(Arutz Sheva’s North American desk is keeping you updated until the start of Shabbat in New York. The time posted automatically on all Arutz Sheva articles, however, is Israeli time.)