The only predictable outcome of yesterday's terror atrocity in Westminster was the appearance of Nigel Farage on the US television networks, urging Americans to link the attack to failures in immigration controls.

In a vacuum of facts and intelligence, the former Ukip leader told Fox News that the events unfolding around Parliament should help remind Americans that lax immigration rules were an "open door inviting terrorism."

Farage was not alone in a cynical rush to judgment. Across the other side of the world, Australia's One Nation leader Pauline Hanson responded to the attack by asking Australians to use the hashtag #PrayForAMuslimBan.

Today we have been told that the attacker was British-born and already known to the security services.

The willingness of politicians to make political capital out of terror shows how entrenched the kneejerk debate on counterterrorism policy has become.

Reaction from the scene for terrorist attack at Westminster

While Farage and Hanson were repeating their tired old lines about immigration, Islamic activists were putting the other side of the debate.

Cage, the human rights group which once infamously branded Jihadi John a "beautiful young man", released their own statement warning the government about the dangers of enacting "reactionary policies".

In the next few days politicians, advised by counterterrorism experts, will turn their attention to how to combat home-grown terrorists who answer the call to jihad by using their cars and kitchen knives to carry out atrocities.

There will be no doubt demands for more armed police officers, greater surveillance and powers to tap our phones and computers.

But the horror of yesterday's terror attack on Westminster exposed Londoners to what the citizens of Paris, Berlin and Nice already know: that in the current, febrile climate of violent Islamist terrorism there is no defence against those who are committed to killing.

Where once the Kalashnikov and the bomb were the terrorists’ weapons of choice, they have been ditched for the low-tech car and knife.

In 2015, well-armed terror cells closely controlled from inside Syria were able to bring carnage to the streets of Paris and Brussels in carefully planned attacks.

The hardline counterterrorism response across Europe was slow but effective, making it difficult for terror cells to organise themselves within the Muslim communities where they had sought shelter.

But this has isolated many young Muslims who have removed themselves from mainstream society so that they have become easy prey to online radicalisers.

Woman fell into Thames after being hit by car, now in stable condition

Security services across Europe have discovered very quickly that there is little they can do to stop a lone wolf terrorist who keeps his deadly plans to himself. In Britain, we have already thwarted 13 terror plots since 2013.

All that MI5 and the police can say for sure is that there will be more attacks like that witnessed in Westminster yesterday.

So the choice is as stark as it is clear: we either accept this new nihilist reality or find radical ways to end the daily threat of terror.

Some have argued that we should permit young Muslims who wish to follow their dreams of jihad to leave Britain for Syria. During the Bosnian war in the early 1990s hundreds of British Muslims were allowed to join the ranks of the mujahideen without harassment from the security services. The vast majority fought and quietly returned to the UK where they peacefully carried on with their lives.

But the security services argue that the nature of the conflict in Syria and Iraq is markedly different because groups like Isis are targeting the West as part of their war. By allowing British Muslims to travel to Syria they fear they will acquire gun and bomb training and then return to Britain to carry out mass attacks, similar to those witnessed in Paris and Brussels.

This containment policy presents its own domestic security issue as hundreds of Muslims now find themselves trapped in the UK, frustrated that they can't travel to Islamist and rebel strongholds and perform what they regard as their religious duty.

In pictures: Westminster attack Show all 9 1 /9 In pictures: Westminster attack In pictures: Westminster attack An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London PA wire In pictures: Westminster attack MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner Reuters In pictures: Westminster attack Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

Britain's zero-tolerance policy on Syria has filled our prisons with young jihadists already marginalised and isolated inside mainstream society. In jail, they associate with like-minded Muslims which only helps to radicalise them further.

When they leave prison, they often pose more of a risk to security than they did before they were arrested.

The resources of the security services are being stretched to the limit as they try to keep tabs on an estimated 3,000 extremists living in the UK.

Across Europe, governments are experimenting with more radical policies. France has discussed re-establishing a Devil’s Island for jihadis. Germany, which has been at the vanguard of enlightened approaches to the terror threat by opening its doors to refugees, has now brought in measures to deport German citizens suspected of terrorism to countries where their parents were born.

In Britain, we must be careful not to let one terror incident end centuries of hard-won universal rights.