The terror threat level in the UK has been raised from substantial to severe, the UK Home Secretary Theresa May has announced, meaning that an attack on the UK is "highly likely" based on developments in Syria and Iraq, where an Islamic militant group has waged a violent campaign.
Meanwhile, UK Prime Minister David Cameron has promised his government will bring new legislation to allow authorities to strip passports from those who travel to foreign countries to fight with jihadists.
In a live televised statement, he said the root cause of an increased terror threat to the UK stems from a "poisonous ideology of Islamist extremism."
At least 500 people have travelled from the UK to fight in Syria and potentially Iraq, Cameron revealed.
While May reassured UK citizens that there is no intelligence evidence to suggest an attack is "imminent," she said the UK faces what she describes as a “real and serious threat” from international terrorism.
She advised members of the public to remain vigilant.
“Severe” is the second highest of five potential threat levels, of which the highest is “critical, meaning an attack is expected imminently.
“The increase in the threat level is related to developments in Syria and Iraq, where terrorist groups are planning attacks against the west,” May said in an official statement.
"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.
"We have already taken steps to improve our powers and increase our capabilities for dealing with the developing terrorist threats we face. That process will continue and the British public should be in no doubt that we will take the strongest possible action to protect our national security," May said.
These developments come a day after the head of British Police (Scotland Yard) called for a major overhaul of the UK's counter-terrorism laws and powers.
Sir Bernard Hogan-Howe said in a radio interview that urgent action is needed because of what he called a “growing threat and the risk of a further rapid influx of dangerous militants into the (UK's) capital”.
He believes the Islamic State organisation in Syria and Iraq poses a very serious threat to the UK.
Sir Bernard said Britons who have travelled to Syria and Iraq numbered “at least 500 to 600 and about 250 had already returned”.