LONDON, United Kingdom – The Director General of the British intelligence service MI5 has warned the risk to the UK from terrorism is the highest he has ever seen because of Islamic State (IS) terrorists. Andrew Parker said IS was planning “mass casualty” attacks in the UK and despite having thwarted six major attacks MI5 cannot guarantee they will detect every plot.

Mr Parker said terrorists have a “greater ambition” than at any other time during his 32-year career, a period that includes some of the worst attacks from Irish Republicans. He went on say the threats had no sign of abating, but instead the country may not yet have seen the “high water mark” of the terrorists ambitions.

In his third public speech since being appointed the intelligence boss said Britain was now facing a three-pronged threat: at home, overseas and online. He also warned that Al-Qaeda had not gone away, despite the public perception that IS had replaced them.

Parker revealed for the first time that his service believed there were 3,000 jihadis who posed a “substantial challenge” to the UK. He said: “This year we have seen strong signs of greater ambition for mass casualty attacks by Isil.”

He continued: “More than 750 people from this country have traveled to Syria to join extremist organisations and join in the fighting. The growth of the threat shows no sign of abating.”

Mr Parker went on to claim the actions of the American traitor Edward Snowden had made the work of his service harder. He claimed Snowden had handed “our adversaries an advantage” by exposing how technology was being used to gather intelligence on them.

“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 said.

“It uses the full range of modern communications tools to spread its message of hate, and to inspire extremists, sometimes as young as in their teens to conduct attacks in whatever way they can.

“The speed at which the process of radicalization can occur online, and the emphasis on relatively low sophistication but nevertheless potentially deadly plots, are two major challenges that flow from Isil’s mode of operation.” He went on to describe new technology as an "enormous challenge" to MI5, because terrorists could communicate with the wider world without being seen.

He said: "Our ability to access and analyse data is more important than ever before.. We also need to draw upon a range of wider tools to mitigate the challenges. 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."

The government is proposing a series of new laws to intercept communications, but the plans face a strong challenge from civil liberties campaigners. The home grown terrorists are almost exclusively from immigrant communities, a problem that could get worse if the government allows mass immigration from war torn Islamic countries like Syria.