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MI5 Director General Andrew Parker has warned: “We will find and stop most attempts to attack us, but not all.”

The biggest threat comes from 400 or so Brits who returned from Syria and Iraq after fighting for Islamic State.

Mr Parker added: “The speed at which radicalisation can occur online and the emphasis on relatively low sophistication but nevertheless potentially deadly plots, are two major challenges.

“Meanwhile threats from al-Qaeda have not gone away and terrorists continue to aspire to mass casualty attacks.

“All of this means the threat we are facing today is on a scale and at a tempo I have not seen before.”