David Cameron has said today that a terror attack on the UK is now 'highly likely.'
The government has raised the official threat level from substantial to severe amid growing fears over the number of extremists returning to the UK from Iraq and Syria, where Islamic State (IS) have seized large swathes of territory.
Security forces have stated that there is no intelligence to suggest a terror attack is imminent, but the UK remains a target for the extremist militants.
At a press conference at Downing Street, the Prime Minister said that IS posed a "greater and deeper threat to our security than we have known before.
"We could be facing a terrorist state on the shores of the Mediterranean and bordering a NATO member," he said.
"My first priority as Prime Minister is to make sure we do everything possible to keep our people safe. The ambition to create an extremist caliphate in the heart of Iraq and Syria is a threat to our own security here in the UK.
"The terrorist threat was not created by the Iraq war 10 years ago. It existed even before the horrific attacks on 9/11, themselves some time before the war.
"This threat cannot be solved simply by dealing with perceived grievances over Western foreign policy. Nor can it be dealt with by addressing poverty, dictatorship or instability in the region - as important as these things are.
"The root cause of this threat to our security is quite clear. It is a poisonous ideology of Islamist extremism that is condemned by all faiths and faith leaders."
He stressed that the Government had already taken steps to counter the threat of jihadists returning to Britain to commit atrocities, but recognised that it had become clear that there was still a need to fill 'gaps in our armoury'.
He also announced new legislation to make it easier to remove extremists' passports if there are concerns they will travel to the Middle East to join IS.
Government officials believe at least 500 Britons have gone to fight in Syria and potentially Iraq.
Home Secretary Theresa May had earlier stated that while there is no information to suggest an attack is imminent, the public should be vigilant.
"We face a real and serious threat in the UK from international terrorism. The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West.
"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. We face a real and serious threat in the UK from international terrorism.
"I would urge the public to remain vigilant and to report any suspicious activity to the police."
It is feared that IS terrorists may target high profile events in the UK.
Haras Rafiq from counter-extremist think tank the Quilliam Foundation, said: "Next week we have a very important Nato conference in Wales. It would be right to suggest that ISIS may well want to target high-profile targets like that.
"The danger is also from a wider aspect in terms of European fighters. We talk about Britain having a large number of fighters out there, but per captia Belgium is the worst offender. The threat is not just from Britain it's from a wider European perspective."
Police forces say they will increase their patrols in response to the raising of the terror threat and have advised transport hubs and sports stadiums to increase their vigilance and security checks. Airports will also be subject to the heightened threat level increase.
National Policing Lead for Counter-Terrorism, Assistant Commissioner Mark Rowley, urged the public to report any suspicious activity. "We need communities and families to bring to our attention anyone they perceive may be vulnerable, a danger or escalating towards terrorism," he said. .
Meanwhile, the White House has said it does not expect to raise the US terror threat level.
Anyone with information is urged to contact the Anti-Terrorist Hotline on 0800 789 321.