Britain is experiencing a terrorist offensive which is as unrelenting as it is unprecedented, drawing from a growing pool of Islamist extremists, and with some murderous plots taking just a few days from inception to the point of attack, the head of MI5 has warned.

Andrew Parker, the director general, said he had never faced such a high tempo of attempted and successful bombings, shootings and stabbings in his 34 years of working in the Security Service.

With jihadis returning from fighting in Isis ranks in Syria and Iraq, and the internet providing motivation and material, the pace of violence shows no sign of falling.

Charting the scale of the problem, Mr Parker said that MI5 and police have stopped seven attacks by Islamists in the past seven months. 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.

There are, at present, 500 live operations under way targeting 3,000 people with 20,000 more who have been on the counter-terrorism radar and others who are not even known to the law agencies.

Mr 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 today: “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.

How do the UK's terror threat levels work?

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”.

The internet providers had a duty to help protect public safety, Mr Parker said. He did not want to dwell, he said, too much on specific matters such as encryption, which have been used by terrorists to plan and communicate, arguing there needs to be an element of partnership on this vital issue between Government and private industry.

Speaking at a location in central London, the director general said: “The threat is multidimensional, evolving rapidly, and operating at a scale and pace we have not seen before. We have seen a dramatic upshift in the threat this year. It’s the highest tempo I have seen in my 34-year career. Today there is more terrorist activity, coming at us more quickly, and it can be harder to detect.

“The threat is more diverse than I’ve ever known. Plots developed here in the UK, but plots directed from overseas as well. 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 victory 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.”

Andrew Parker led the investigation into 7/7 attacks

To add to the security threat, hundreds of British Muslims who had joined Isis in the Middle East are expected to be heading home. More than 800 had made the journey out, said Mr Parker, and around a hundred of them are believed to have been killed.

A relatively small number have returned, but, as Isis faces defeat, he added they are now advising would-be fighters to carry out their jihad in other countries.

The 10 developed countries suffering the most deaths from terrorism Show all 10 1 / 10 The 10 developed countries suffering the most deaths from terrorism The 10 developed countries suffering the most deaths from terrorism 10: Greece Greek Presidential Guards, or Evzones, take their place for a review by US President Barack Obama and his Greek counterpart Prokopis Pavlopoulos in Athens, Tuesday, Nov. 15, 2016. President Barack Obama arrived in Greece Monday morning on the first stop of his final foreign tour as president, the first visit to Greece by a sitting U.S. president since Bill Clinton in 1999 trip. (AP Photo/Lefteris Pitarakis) AP The 10 developed countries suffering the most deaths from terrorism 9: Denmark Shutterstock The 10 developed countries suffering the most deaths from terrorism 8: Austalia The incident allegedly took place in a primary school in Sydney Getty Images The 10 developed countries suffering the most deaths from terrorism 7: Sweden Ola Ericson/imagebank.sweden.se The 10 developed countries suffering the most deaths from terrorism 6: Mexico People talk to relatives at a section of the wall separating Mexico and the United States, as photographed from Playas Tijuana, in Tijuana, Mexico, November 6, 2016. REUTERS/Stringer FOR EDITORIAL USE ONLY. NO RESALES. NO ARCHIVES. Reuters The 10 developed countries suffering the most deaths from terrorism 5: Germany Afghan asylum seeker Shakira Sarwari, 27, and her two children, Mohammed, 17 months, and Setayesh, 7, are taken off a Munich-bound train in Salzburg, Austria, by German police Anthony Faiola/The Washington Post The 10 developed countries suffering the most deaths from terrorism 4: Israel Israeli Prime Minister Benjamin Netanyahu, attends the weekly cabinet meeting in Jerusalem, Sunday, Nov. 13, 2016. (Ronen Zvulun, Pool via AP) AP The 10 developed countries suffering the most deaths from terrorism 3: US US police have shot 698 people in 2016 so far. Four times more black than white people have been shot Getty The 10 developed countries suffering the most deaths from terrorism 2: France French policemen and security officers stand guard at the entrance of the Bataclan concert hall in Paris on November 12, 2016, a few hours before the reopening concert by British musician Sting to mark the first anniversary of the November 13 Paris attacks. Rock star Sting reopens the Bataclan on November 12, the revered Paris concert hall where jihadists massacred 90 people, with a hugely symbolic show to mark the first anniversary of France's bloodiest terror attacks. Scores of survivors of the Bataclan assault -- the worst of the gun and suicide attacks across the city that night which left 130 dead -- will attend the concert, the dominant event in a weekend of otherwise low-key commemorations. / AFP PHOTO / PHILIPPE LOPEZPHILIPPE LOPEZ/AFP/Getty Images Getty The 10 developed countries suffering the most deaths from terrorism 1: Turkey AP

“This upshift is driven by the murderous strategy of Daesh [Isis] and online propaganda. Daesh directing mass casualty plots, Daesh encouraging particular extremists in the UK to kill, Daesh inspiring attacks generally, by lone actors or small groups or anyone who will listen to their poisonous message,” said Mr Parker. “We’ve also seen that terrorism breeds terrorism. Would-be attackers take encouragement from the acts of others and can be galvanised into taking action themselves, acts of violence become normalised in their twisted thinking.”

Technological advance was inevitable and welcome, Mr Parker said. “But an unintended side effect is that these advances also aid the terrorists. Whether it’s the ease of online purchasing, social media content, or encrypted communications, addressing these challenges is about partnerships and ethical responsibility,” he said.

“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 terrorist planning beyond the sight of MI5. Some helpful action is being taken. But there is a challenge of pace, volume and reach as these technologies continue to develop so rapidly. We have a shared responsibility to do all that we can to prevent terrorist exploitation of internet-carried services.”

Asked about the security implications of a possible hard Brexit, Mr Parker stressed that cooperation with partner agencies in Europe will continue: ”We don’t just stand with our European colleagues, we work with them. We share intelligence. We run joint operations, every single day.

“Only last week I met again with my counterparts from 30 European security services as we decided on the next stages of collective action. It might help illustrate how close our working is if I say that we have a joint operational centre, which happens to be based in the Netherlands, where officers from across our services are working alongside each other every day in joint facilities with shared data in joint endeavour against the terrorists.

“This delivers real results. Together we have stopped attacks. More than a dozen terrorists are in custody today who might not otherwise have been found in time.” The 12 suspects, it is believed, are being held in Europe. The plots some of them have been allegedly involved in have a British connection.

MI5 is doing its utmost and achieving success against terrorists, said Mr Parker. “I’ve always been clear that we can’t hope to stop everything. But I can tell you that MI5 and our partner agencies are bringing the full weight of our growing capabilities to counter this new intensity of threat.

“Day in and day out we are identifying and disrupting threats: stopping terrorism. Our response is unrelenting. When an attack happens we are determined, using the harsh light of hindsight, to squeeze out every last drop of learning so that we can be the very best we can be, now and in the future.

“The women and men of MI5 are of course members of the public, who are drawn from across the society that they protect. They get up and come to work every single day to make terrorist attacks less likely and to keep the country safe.