The UK’s terror threat level has been raised from “substantial” to “severe”, which means the government believes an attack is highly likely.
According to the BBC, the move is in response to conflicts in Iraq and Syria, Home Secretary Theresa May says.
It is the second highest of five possible UK threat levels.
David Cameron promised new legislation would make it easier to take passports from those travelling abroad to fight.
The home secretary already has the power, under the Royal Prerogative, to withhold a passport if it is in the public interest to stop somebody travelling.
That power has been used 23 times between since April 2013 to stop people travelling abroad for alleged terrorist-related or criminal activity.
In a Downing Street press conference on Friday, the prime minister said Islamic State (IS) extremists – who are attempting to establish a “caliphate”, or Islamic state – represented a “greater and deeper threat to our security than we have known before”.
The raised threat level may not lead to visible signs of change on the streets – but it is a sign of the increased concern and security activity behind the scenes involving all of the UK’s intelligence and security bodies.
He said that “learning lessons from the past doesn’t mean there isn’t a place for our military” in combating the threat, but did not commit to any military action.
He added the “threat is growing” from Britons travelling to fight with IS, saying at least 500 people had travelled from the UK “to fight in Syria and potentially Iraq”.
Assistant Commissioner Mark Rowley, the Association of Chief Police Officers’ lead for counter-terrorism, said security and protection measures were being increased following the raised threat level.
Efforts are continuing to identify a suspected British jihadist who appeared in IS footage of the killing of US journalist James Foley.
The last time the level was this high was between January 2010 and the summer of 2011. This may have been linked to attempts by an al-Qaeda affiliate to smuggle bombs on to planes heading out of the Middle East.
The highest level is “critical”- meaning an attack is expected imminently. Officials have twice put the country on such an alert – in 2006 after the discovery of liquid bombs aimed at airliners and then the following year when extremists attempted to bomb Glasgow Airport and London’s West End.
In other words – if security chiefs had knowledge of a clear threat they could not contain, the level would already be one notch higher.