Andrew Parker, MI5 chief, has revealed that the Islamic State (Isis) terrorists are planning to carry out "mass casualty attacks" in Britain and the country's domestic security agencies should be prepared to make "difficult choices" to defend the country from homegrown jihadis.

In a rare public speech, MI5 chief said that the Security Service thwarted six terror attacks in the past year alone in UK. "We can never be confident of stopping everything," he warned. 

"The death of 31 British nationals in the Sousse attacks in June was an appalling reminder of the threat."

In only his third speech since taking up the job in 2013, Parker said that the "war on terror" was getting complex as the terrorists are now using a "bewildering array of devices and digital platforms" to plot attacks.

He also publicly, for the first time, admitted that in some instances MI5 even hacked into computers of suspects to access their communication networks.

Despite all this Parker assured: "It may not yet have reached the high water mark." He reiterated that while the threat from Isis was growing, Al Qaeda still poses a danger to peace.

In the Lord Mayor of London's annual defence and security lecture, the MI5 top-boss said that Isis was exploiting complex web tools to radicalise youth including teens to carry out terror acts in their own countries.

To this, he added that the Internet firms have an "ethical responsibility" to help MI5 tackle terrorism.

"This year we have seen strong signs of greater ambition for mass casualty attacks by Isis," he said.

"More than 750 people from this country have travelled to Syria to join extremist organisations and join in the fighting. The growth of the threat shows no sign of abating."

[Read full text of speech here]

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