London: The UK’s terror threat level has been raised from “substantial” to “severe” in response to conflicts in Iraq and Syria, Home Secretary Theresa May says.

The new alert level rates the risk of an attack on the UK “highly likely”, although Mrs May said there was no evidence to suggest one was “imminent”.

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.

‘Gaps in armoury’

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”.

PM David Cameron: “We need to fill the gaps in our armoury”

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.

And efforts are continuing to identify a suspected British jihadist who appeared in IS footage of the killing of US journalist James Foley.