Andrew Parker, head of the MI5 British homeland security agency, warned on Friday that Islamic State (ISIS) terrorists are planning to launch a mass casualty terror attack in the UK.

During the Lord Mayor of London's annual defense and security lecture, Parker revealed that no fewer than six planned attacks were thwarted by MI5 in the last year alone, aside from another seven plots targeting British nationals abroad, reports The Telegraph.

"More than 750 extremists from this country have traveled to Syria, and the growth in the threat shows no sign of abating," he revealed.

“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," he added, using an alternate acronym for ISIS.

Giving a specific warning, he noted that "this year we have seen greater ambition for mass casualty attacks," and aside from ISIS, Al-Qaeda terrorists are likewise planning huge attacks in the UK.

"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," said the veteran security official, noting on his 32-year career.

In response, Parker said no less than four fifths of MI5's 4,000 members are focused on preventing terror attacks, with ISIS becoming a major focus of their attention.

In addition to allocating huge manpower resources to the threat, he revealed for the first time that MI5 has been hacking terrorists online with a new "toolbox" of methods.

"This includes the ability to conduct operations online and to mount IT attacks (known as equipment interference), under a warrant authorized by the Home Secretary, against terrorist networks, so that we can access their communications.”