The United Kingdom's intelligence services are facing an "intense" challenge from terrorism, the head of MI5 has warned.
He noted that Britain is running more than 500 live operations targeting 3,000 individuals involved in extremist activity in some way with 20,000 more, who have been on the counter-terrorism radar and others who are not even known to law agencies.
Director General of MI5 Andrew Parker said there was currently "more terrorist activity coming at us, more quickly" and that it can also be "harder to detect".
Speaking in London, Parker said the tempo of counter-terrorism operations was the highest he had seen in his 34-year career at MI5.
The UK has suffered five terror attacks this year, and he said MI5 staff had been "deeply affected" by them.
Twenty attacks had been foiled in the last four years, including seven in the last seven months, Parker revealed, adding that all were related to what he called Islamist extremism.
Britain has experienced five attacks using knives, cars and bombs since the beginning of 2017, four in London and one in Manchester.
Twenty major acts have been detected in the past four years and 379 suspects have been arrested in the first six months of this year, Parker stressed.
Parker recalled that he had said in a speech two years ago that despite a string of successful operations he feared “that we had not yet reached the high-water mark”.
He said Tuesday: “Sadly that has proved to be the case. Islamist terrorism is an acute and enduring challenge that requires a sustained and comprehensive approach.”
The internet has provided terrorists with access and immediacy. “They can go online to get explosives and learn how to build a bomb”, said the director general.
As a result, they can “accelerate from inception to planning to action in just a handful of days, exploiting safe space online which can make it harder to detect and gives us a smaller window to intervene”.
MI5 is under pressure to demonstrate its effectiveness after four Islamist terrorist attacks escaped its detection this year.
Parker’s speech to specialist security journalists on Tuesday was his chance to frame the debate about Britain’s battle against Islamist terrorism at a time when the agency’s staff numbers are already expanding from 4,000 to 5,000, according to The Guardian.
This month the government will receive reports on whether chances to thwart the atrocities were missed and what lessons could be learned.
Ministers and the National Security Council wanted independent oversight of the review, in essence not allowing MI5 or counter-terrorism police to assess themselves.
Oversight is being provided by the barrister David Anderson QC, a former government appointment as independent reviewer of terrorism legislation.