The head of MI5 today warned Britain is facing the biggest terror threat of his 34 year career - as he said tech giants have an 'ethical responsibility' to crack down on extremists.

Andrew Parker said extremists are mounting deadly terror attacks with just a few days of planning as the UK sees a 'dramatic' jump in the scale and pace of the threat.

In his annual 'state of the union' assessment of the threat facing the UK, he said extremists are exploiting 'safe spaces' online, hindering intelligence efforts to root them out.

And he issued a fresh challenge to technology firms, saying they have an 'ethical responsibility' to help governments confront the threat.

His stark warning comes after Britain was hit by five terror attacks this year - killing dozens and injuring hundreds more.

Head of MI5 Andrew Parker pictured in London today, said Britain is facing the biggest terror threat of his 34 year career in the intelligence services

In his first substantial remarks since the wave of carnage, this year, Mr Parker detailed the 'acute and enduring' challenge posed by Islamist terrorism.

Twenty plots have been foiled over the past four years, including seven in the last seven months.

Speaking in central London today, he said: 'We've seen a dramatic up-shift in threat this year.

'Today there is more terrorist activity, it's coming at us more quickly and it can be harder to detect.'

Security services and police are operating at a scale which is 'greater than ever before', he said.

Mourners leave candles in tribute to those killed after the Manchester terror attack. Britain has been hit by a wave of terror attacks this year, killing dozens and injuring hundreds more

They are probing well over 500 live operations involving around 3,000 individuals, he revealed.

And the top spy also warned that as Islamic State - also known as ISIS or Daesh - are on the run in Syria, British intelligence services have to assess the risk posed by those fleeing the war in the Middle East.

TECH FIRMS HAVE RESPONSIBILITY TO CONFRONT TERROR, MI5 CHIEF WARNS Technology companies have an 'ethical responsibility' to help confront the unprecedented terrorist threat, the head of MI5 has said. Andrew Parker called for companies to work in partnership with governments to stop violent extremists exploiting their services. And he warned that some extremists were finding 'safe spaces' online to plot carnage. He said: 'Addressing these challenges is about partnerships and ethical responsibility. 'No company wants to provide terrorists with explosive precursors. 'Social media platforms don't want to host bomb-making videos and communications providers don't want to provide the means of terrorists' planning beyond the sight of MI5.' Mr Parker, the Director General of MI5, said 'technology is not the enemy'. But he warned an 'unintended side effect is that these advances also aid the terrorists'. He said: 'I believe there is a responsibility on the companies that offer those services to help governments be able to stop the worst excesses of human criminal behaviour.' He added: 'For all the companies, and it's a wide field, there are many large companies and many more smaller ones, I think the same principle applies. 'There must be an ethical responsibility to help with these problems.' Advertisement

He said there is a 'growing pool' of more than 20,000 people looked at as part of previous terror investigations.

Mr Parker also noted that there will be some violent extremists 'not yet known to us at all'.

He said the shift is being driven by the 'murderous strategy' of IS and online propaganda, and warned that the threat is 'more diverse than I've ever known'.

'Plots developed here in the UK but plots directed from overseas as well,' he said.

'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.'

He said attacks can sometimes accelerate 'from inception through planning to action in just a handful of days'.

He added: 'This pace, together with the way extremists can exploit safe spaces online, can make threats harder to detect and give us a smaller window to intervene.'

Among the plots seen in recent years some have had a 'very short life from first inception to intent to act', Mr Parker said.

'Some of them would be measured in days, at the extreme,' he added.

While IS is 'rapidly losing ground' in its heartlands in Syria and Iraq, tackling it as a movement will require 'sustained international focus for years to come', the Director General of MI5 said.

He made clear that counter-terror agencies 'can't hope to stop everything', acknowledging that 'attacks will occur sometimes'.

But he said the vast majority are being found and stopped and a 'perfect standard of 100 per cent' is not achievable.

Striking a defiant tone, he described the response to the threat as 'unrelenting'.

Armed police surround terrorist Khalid Masood in parliament's New palace Yard after he ploughed his car into pedestrians in nearby Westminster Bridge before leaping out and stabbing to death PC Keith Palmer

Armed police clad in black masks and carrying machine guns were deployed onto the streets of London after the London bridge terror attack - in which three terrorists drove their car into innocent bystanders enjoying an evening out before leaping out and stabbing them

Mr Parker said: 'Those that wish our country harm can expect to meet MI5 and the police. They will face the full force of the law and be brought to justice.'

His comments come after five terror attacks have brought murder to the streets of Britain this year in a dramatic escalation of the threat facing the country.

Some 22 people - including many children - were killed when a suicide bomber blew himself up at an Ariana grande concert in Manchester in May.

Less than two weeks later three jihadis struck in London Bridge, killing eight people and injuring dozens more when their white van drove into pedestrians n a busy evening.

They then leapt out and stabbed passers by before being shot dead by police in just eight minutes.

The heart of government was attacked in March when Khalid Masood, 52, drove his car into pedestrians in Westminster Bridge before jumping out and stabbing PC Keith Palmer by Parliament's gates.

While ten people were injures and one died after a man drove his van into Finsbury Park mosque in June.