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Updated: Jan 09, 2015 15:23 IST

The al Qaeda network is plotting a terrorist strike in Britain to inflict mass casualties similar to the newsroom massacre at a satirical French weekly that killed 12 people, Britain's spy chief has warned.

Andrew Parker, the Director General of MI5 Security Service, warned that the threat of a "mass casualty attack" was growing and that intelligence pointed to the existence of specific plots.

He said while British police and MI5 had "stopped three UK terrorist plots" in recent months, the threat of a terror attack in the UK is increasing and the security services cannot be expected to stop every plot.

"We cannot be complacent. Although we and our partners try our utmost we know that we cannot hope to stop everything," he said. "The going is getting tougher but there are good reasons for confidence in our enduring ability to respond. It's well-understood that we can't guarantee to stop everything, but we continue to strive to get as near to that as we can."

Security has been stepped up at British ports and armed police are on patrol at the Eurostar terminal at London's St Pancras station after 12 people were slaughtered by al Qaeda gunmen at 'Charlie Hebdo' magazine's office in Paris on Wednesday.

MI5 officers have increased surveillance of British fanatics who they fear may launch copycat attacks. 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. A "significant proportion" of the 600 Britons now believed to have travelled to Syria have joined Islamic State (IS) militants.

Parker warned that despite its "medieval tactics", IS was a "terrorist phenomenon of the modern age", making "full use of the modern social media and communications methods through which many of us now live our lives".

"Wherever we lose visibility of what they are saying to each other, so our ability to understand and mitigate the threat that they pose is reduced," he added.