The head of Britain's security agency says terrorist threat levels are the highest he has ever seen, and his agency has thwarted six terror plots in the UK and seven abroad in the last 12 months
Islamic State terrorists are planning mass-casualty attacks in Britain, the head of MI5 has warned.
Andrew Parker, director general of the security agency, said threats from homegrown jihadis who want to fight for the militant movement showed no sign of abating.
He also publicly admitted for the first time that MI5 had to carry out computer hacking attacks against terror networks to crack their communications.
Delivering the Lord Mayor of London's annual defence and security lecture, he said the current level of threat was the highest he had seen in a career spanning 32 years. In the past 12 months his agency has thwarted six terror plots in the UK and another seven abroad.
Parker said four-fifths of the 4,000-strong agency's resources were directed at stopping terrorist attacks, with an increasing proportion of them linked to Syria and Islamic State of Iraq and the Levant (Isil).
He said, ''More than 750 extremists from this country have travelled to Syria, and the growth in the threat shows no sign of abating.
''We are seeing plots against the UK directed by terrorists in Syria; enabled through contacts with terrorists in Syria; and inspired online by Isil's sophisticated exploitation of technology.''
Britons are being rapidly radicalised online and then encouraged to carry out low tech but deadly attacks.
He said, ''On top of that, in a range of attacks in Europe and elsewhere, this year we have seen greater ambition for mass casualty attacks.''
While the rise of Isil had dominated the headlines in the past 12 months, he said there was still a threat from al-Qaeda terrorists who are also planning massive attacks in the UK.
He said, ''All of this means that the threat we are facing today is on a scale and at a tempo that I have not seen before in my career.''
He went on: ''We have thwarted six attempts at terrorist attacks in the UK in the last year, and several plots overseas.
''It may not yet have reached the high water mark, and despite the successes we have had, we can never be confident of stopping everything. The death of 31 British nationals in the Sousse attacks in June was an appalling reminder of the threat.''
The scale of the threat meant MI5 had to update its ''toolbox'' of methods to fight terrorists, including using computer attacks.
''This includes the ability to conduct operations online and to mount IT attacks (known as equipment interference), under a warrant authorised by the home secretary, against terrorist networks, so that we can access their communications,'' he said.
Defending the agency's ability to access communications data, he said, ''We use these tools within a framework of strict safeguards and rigorous oversight, but without them we would not be able to keep the country safe.
''As I have said before, we do not, and could not, go browsing at will through the lives of innocent people.''