MORE than 300 jihadists are on the loose in Britain amid warnings that a Brussels-style attack is sure to happen here.
The UK is also on guard against an Islamic State cell thought to be plotting attacks across Europe.
It is believed 800 Britons have gone to fight for ISIS in Syria and Iraq, with up to 360 now returned and seen as a serious, radicalised threat.
Security services are having to prioritise between jihadis who have had their fill of carnage, fanatics bent on bringing war to Britain’s towns and those who will become “sleepers” – biding their time before being activated for a mission by IS.
Meanwhile, any country taking part in the US-led coalition currently bombing IS in Iraq and Syria, is thought to be high on the terror group’s European hit list.
France, which was hit by the Paris terror attacks in which IS killed 130 in November, and Belgium, which is still reeling from Tuesday’s massacre of at least 31 people, have both taken part in coalition air raids.
And Britain has carried out more air raids than any other partner after the United States.
Yesterday, former Labour Home Secretary Lord Reid warned that an attack in Britain was inevitable.
“It will happen because terrorists only have to get through once,” he said.
“You can quote the statistics on how many plots have been foiled, but the terrorists will get through.”
He told the BBC’s Victoria Derbyshire programme: “Politicians ought to be honest with the British people and tell them, ‘This will happen.’”
Lord Reid said terrorists are more likely to focus on “soft targets” like the transport network and crowded public places because the security services had been so successful at preventing “spectacular” plots such as mid-flight attacks on aircraft.
And yesterday a senior counterterrorism officer also admitted a Brussels-style terror attack could happen here.
Deputy assistant commissioner Neil Basu of the UK Counter Terrorism Policing Network, said police and the intelligence agencies had been working “very, very hard” to prevent such an incident occurring.
“We are never complacent. We have got a very fine partnership between the police service and our intelligence agencies that has been forged over decades of fighting terrorism. We have stopped seven plots in the just over 12 months,” he told the BBC Radio 4 Today programme.
“We’ve assisted in disrupting plots overseas as well. So it could happen but we are working at a great pace to make sure that it doesn’t.”
He said authorities in the UK had been preparing for a major attack since the assault on Mumbai in 2011.
He continued: “Paris was a gamechanger for Europe – there is no doubt about it – so we are looking at increasing capacity.
“But we have some of the best capability that has been developed post-Mumbai in terms of armed policing and how we go about stopping these plots from developing in the first place.”
The threat to the UK was underlined yesterday as two London jihadis faced jail for plotting to assassinate soldiers and police.
Medical student Tarik Hassane, 22, dubbed “The Surgeon,” and Suhaib Majeed, 21, planned to gun down victims in drive-by shootings and escape on motor scooters.
The chilling conspiracy is one of scores of similar plots aimed at British targets which have been thwarted by the security services and police since the 7/7 bombings in 2005.
Hassane admitted conspiracy to murder and Majeed was found guilty of the same charge yesterday.
They face long prison sentences when they are sentenced at the Old Bailey next month.
Two other men who supplied guns, ammunition and a silencer for the proposed killings will be jailed with them.
Three of the men reportedly went to the same London mosque as Mohammed Emwazi – dubbed “Jihadi John” – who was killed in a drone attack in Syria last year.
Security chiefs have also admitted they are worried by the prospect of terrorists getting nuclear weapons.
These could include “dirty bombs” laced with radioactive material.
Defence Secretary Michael Fallon said yesterday: “It is obviously a concern that we will see non-state actors with the finance and some of the technical know-how seeking to get hold of nuclear weapons.
“That is why we maintain very strict export control criteria for the technologies involved.”
With the EU’s open borders allowing terrorists to roam across the continent almost at will, security services were yesterday scouring the emerging evidence for a British link to the Brussels attacks.
After the Paris attacks there were claims that the Belgian ringleader, Abdelhamid Abaaoud, visited London and Birmingham in the months preceding the slaughter.
Meanwhile Prime Minister David Cameron tried to play down fears of a bloodbath on British mainland.
A No 10 spokesman said security had been stepped up at a number of locations with the national threat level kept at “severe”, meaning an attack is “highly likely”.
However, Britain’s authorities are battling to keep tabs on the large returning army of jihadis.
Of the 800 who have travelled to join IS, it is believed that 100 – including “Jihadi John” – have been killed fighting with Iraqi and Kurdish forces and in coalition air raids.
Jihadis who have been indoctrinated and have fought and killed in Syria are seen as a much graver threat than “lone wolves” radicalised over the internet.
The scale of the problem was revealed by Europol – the European police agency last year.
It said up to 5,000 Europeans have flocked to the black jihadi banner, creating a huge potential army of terrorists.