Al-Qaeda is planning a Paris-style terrorist atrocity against Britain, according to the head of MI5.
Andrew Parker, the Director General of the Security Service, warned that the threat of a “mass casualty attack” was growing and that intelligence pointed to the existence of specific plots.
Security was stepped up on Wednesday at British ports, and armed police were put on patrol at the Eurostar terminal at London’s St Pancras station.
Mr Parker warned that although three terrorist plots had been foiled in recent months, it was almost inevitable that one would eventually succeed.
MI5 officers have increased surveillance of British fanatics who they fear may launch copycat attacks, after 12 people were slaughtered by al-Qaeda gunmen at Charlie Hebdo magazine in Paris.
Intelligence has shown that the Syrian arm of a resurgent al-Qaeda is planning similar outrages against the UK, possibly by British jihadists who have already returned from fighting in Syria or Iraq.
They include plans to blow up a passenger jet; Mumbai-style shootings in crowded places; or even hit-and-run attacks using vehicles.
Mr Parker said the number of random “crude and potentially deadly” plots from “lone wolf” extremists was increasing.
In a stark warning, he said: “Although we and our partners try our utmost, we know we cannot hope to stop everything.”
It is feared that a lack of cooperation from internet companies means that there is a risk of terrorists slipping through the net because MI5 cannot track them.
Mr Parker warned that terrorists may soon be able “to operate beyond our reach” as he renewed calls for enhanced access to digital communications.
On Wednesday night, the net was closing on the Charlie Hebdo killers, as hundreds of armed French police officers searched a vast expanse of woodland outside Paris.
Chérif and Saïd Kouachi, French brothers aged 32 and 34 of Algerian descent, were thought to be hiding out in the Forêt de Retz, east of Paris towards Reims, after holding up a petrol station and later abandoning their car and escaping on foot.
Armed police were stationed at the “gates” to Paris on its eastern ring road, in case the heavily armed duo had plans to return to the capital for a second attack.
Earlier, in a separate shooting in southern Paris, a 27-year-old police officer, Clarissa Jean-Philippe, was killed attending a traffic accident. A traffic warden was seriously injured in the shooting. The gunman, said to be clad in black, escaped. But on Wednesday police said they had arrested two men.
Nine others were being held in connection with the investigations.
As night fell, the lights of the Eiffel Tower were switched off in memory of the victims, as the surviving members of Charlie Hebdo vowed to continue their work, by publishing a special tribute edition of one million copies – up from their normal run of 60,000 – next Wednesday, with contributions from cartoonists around the world.
Speaking at a business conference in Manchester, the Prime Minister said: “Everyone’s thoughts will be with the French people and will be thinking about the appalling events that happened yesterday in Paris.
“These are a challenge to our security because we have to fight this terrorism with everything we have.
“But they are also a challenge to our values and … when it comes to security we must use every part or vigilance and security we have.” When it comes to values, he said the best way to protect them was to “repeat our belief” in them. He added: “We stand with them today in defending the values we all hold dear.”
Mr Parker said the attack was a “terrible reminder” of the intent of Islamist extremists to harm the West. In only his second speech since being appointed in 2013, he said that while Islamic State of Iraq and the Levant (Isil) was still a key worry, the threat from al-Qaeda remained.
He said: “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.”
It is understood that the threat to the UK comes from the “Khorasan” cell, an al-Qaeda offshoot made up of veteran jihadists sent to Syria by Ayman al-Zawahiri, the terrorist organisation’s leader. It is likely to include British fanatics.
Speaking to members of the Royal United Services Institute (Rusi) at MI5 headquarters, Thames House, Mr Parker said the terrorist threat to the UK was “complex to combat and unlikely to abate for some time”.
He said it was now believed that about 600 British jihadists were either in Syria or had returned, up from the previous estimate of 500.
He described Isil as a “terrorist phenomenon of the modern age” and said that in recent months three UK terrorist plots which would have led to deaths had been foiled.
Mr Parker also revived demands for the police and security services to have the ability to access more forms of communications when tracking suspects.
Previous attempts to introduce legislation that would force internet companies to store all users’ communications, so it can be monitored if need be, were nicknamed the “snooper’s charter” and failed after opposition from the Liberal Democrats.
Encryption services used by internet companies is also making it easier for terrorists to hide their intentions, and there has been a lack of cooperation with the security services after the Edward Snowden revelations.
The MI5 chief said the growing gap in capability remained his “sharpest concern” and warned: “We increasingly face a world in which those who pose a serious threat may be able to operate beyond our reach.”
He added: “We all value our privacy and none of us want it intruded upon improperly or unnecessarily. But I don’t want a situation where that privacy is so absolute and sacrosanct that terrorists and other who mean us harm can confidently operate from behind those walls without fear of detection.”