As the world reels from the Charlie Hebdo massacre in Paris, the head of MI5 has warned that al-Qaeda is plotting a mass attack on the UK.
Andrew Parker, director general of the security service, told a select group mostly from the Royal United Services Institute that while the Islamic State have been encouraging lone wolf attacks, al-Qaeda are planning a massacre that will target large numbers of civilians.
It's feared that al-Qaeda plan to draw on radicalised British volunteers in Syria to carry out a large-scale atrocity in the UK.
He said: "We know that terrorists based in Syria harbour... ambitions towards the UK - trying to direct attacks against our country, and exhorting extremists here to act independently.
"We still face more complex and ambitious plots that follow the now sadly well-established approach of al Qaeda and its imitators: attempts to cause large scale loss of life, often by attacking transport systems or iconic targets.
"We know, for example, that a group of core al Qaeda terrorists in Syria is planning mass casualty attacks against the West."
UK terror attack 'highly likely'
Parker's comments come as an international manhunt is underway for two suspected gunmen, who on Wednesday (7 January) attacked the offices of satirical magazine Charlie Hebdo, killing 12 people. It is believed the gunmen are affiliated to al-Qaeda in the Arabian Peninsula (AQAP).
The UK's COBRA defence committee has ordered a boost to national security, in light of the massacre in Paris.
Home Secretary Theresa May said: "Following the attacks [in Paris] we took the precautionary step yesterday of increasing security at the French-UK border.
"The UK threat level, which is set by the independent Joint Terrorism Advisory Centre, remains at severe; that means a terrorist attack is highly likely and the public should remain vigilant.
"This morning I chaired COBRA to consider Britain's response to the attacks and our own preparedness to a similar attack."
While there are growing fears of an atrocity on British soil, the increasing number of lone wolf plots aimed at the police and military are also a cause of unease among officers in the UK, who are unarmed.
"It's a huge worry for us," said Sir Peter Fahy, chief constable of Greater Manchester Police.
"There is a particular increased threat to police officers at the moment and there have been a number of attacks foiled in differed parts of the country, but I can tell you that our officers are very concerned about it... they see what happened in France, and clearly we are an unarmed police force but it is the nature of that threat which is worrying but despite that our officers go out there and do their duty."
Extra police have been deployed at British ports and stations serving the Eurostar.