THE head of MI5 has warned that Britain is facing an unprecedented terror threat, with security services at risk of being overwhelmed by "more terrorist activity coming at us, more quickly".
Andrew Parker said he had never seen counter terrorism operations working at such speeds in his 34 years at the UK intelligence agency.
He added that 20 planned attacks on British soil had been foiled in the past four years, seven of them in the past seven months.
All of them had been masterminded by Islamic extremists, he said.
The prolifacy of the plots meant it was becoming "harder to detect" every attack being planned.
MI5 staff were also "deeply affected" whenever an attack is carried out, Mr Parker told an audience in London.
"They get up and come to work every single day to make terrorist attacks less likely and to keep the country safe," he said of his staff.
"They are constantly making tough professional judgements based on fragments of intelligence; pinpricks of light against a dark and shifting canvas.
There have been five terror attacks on the UK this year so far, killing 36 people in total.
In his first substantial remarks since those attacks, Mr Parker detailed the "acute and enduring" challenge posed by Islamist terrorism.
He also issued a fresh challenge to technology firms, saying they have an "ethical responsibility" to help governments confront the threat.
"We've seen a dramatic up-shift in threat this year," he said.
"Today there is more terrorist activity, it's coming at us more quickly and it can be harder to detect."
He highlighted the "short life" of some attack plans, which he claimed could escalate from conception to violence within a matter of days.
He also explained how extremists can exploit "safe spaces" online, making the task of disrupting their activities harder.
When asked directly whether Google, Facebook and Amazon were doing enough to prevent communications among militants, he said: "There is a reasonable expectation, I think, from all of us but also from the public at large that these companies would do what they can to help us deal with these worst excesses."
The intelligence chief said the evolving threat was being driven by the "murderous strategy" of Islamic State and online propaganda, and warned that the risk are "more diverse than I've ever known".
"Plots developed here in the UK but plots directed from overseas as well," he said.
"Plots online. Complex scheming and also crude stabbings. Lengthy planning but also spontaneous attacks.
"Extremists of all ages, gender and backgrounds united only by the toxic ideology of violent hatred that drives them.
"These threats are sometimes now coming at us more quickly - whether crude but lethal attack methods, for example using a knife or a vehicle, or more sophisticated plots, when in today's world terrorists can learn all that they need online to make explosives and build a bomb."
Mr Parker said there were currently 500 live operations involving 3,000 people involved in militant activity.
MI5 has been reviewing the way it uses intelligence on known extremists following the deaths of 22 people killed by suicide bomber Salman Abedi at the Manchester Arena in May.
Abedi was not among the 3,000 people currently under active investigation by MI5, although he was one of around 20,000 people known to have some connection to extremism.
"When an attack happens, we are determined, using the harsh light of hindsight, to squeeze out every last drop of learning," Mr Parker said.
"We are constantly evolving to stay ahead."