A small queue of young men gathered quickly to see the Vietnamese speaker in our team yesterday at our weekly surgery in Harmondsworth immigration detention centre. None spoke English. All arrived clutching similar paperwork. They were victims of torture and likely victims of trafficking, but the Home Office had made a decision to detain them nevertheless.

Some looked under 18; others had such traumatic stories you can but wonder at the sheer courage and dignity of the human spirit that can sustain a person in the face of this much injustice.

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The reasons that had been given by the state for continued detention of these men were incredible, to say the least. They had been told that they had entered the country “in a clandestine manner”, proving that they could not be trusted – but they were trafficked, for pity’s sake, illegally, into slavery, against their will. They were told that the company they had kept demonstrated bad character – referring here, presumably, to the criminality of their torturers.

You couldn’t make this stuff up. And yet, here we have again the routine reasoning of the Home Office in an era of hostility. For this is the fruit of a system that weighs any human story against the sacred cow of immigration enforcement and finds it wanting. This is a system that views all migrants with suspicion, and so there is no room for mercy, no matter how compelling the case.

The treatment of young Vietnamese men, trafficked into slavery, abused and mistreated, then detained for months on end, which we detail in a report published on Friday by the Jesuit Refugee Service, shines a light on the system. As the Guardian revealed earlier this week, the immigration detention system in the UK is full of people who should not be there.

Immigration detention, in spite of government reassurances, is not used in the UK as a last resort. There is no presumption in favour of liberty. Thousands of people are incarcerated every year for the purpose of immigration control, and there is no time limit on how long it can last. Sometimes it goes on for years. One of those we met this week in Harmondsworth had been languishing in detention for 15 months – even though there seems little or no chance of removing him from the UK. His spirit was defiant and his humour still razor sharp, but most others we see are worn down by the detention regime. The toxic mix of fear and uncertainty with the listlessness of captivity destroys mental health and hope.

It destroys families too. Parents are separated from children. Husbands from wives. Friends from community. And its impact lingers long after detention ends. A statistic that will shock many is that about half of those in detention are not removed from the UK but released, after some time, back into the community – only then they are more vulnerable than before, damaged by the state. Life after detention for many is forever framed by the chronic fear that they may once again be arbitrarily detained.

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The Home Office claims to have policies in place to reduce the detention of vulnerable people. But as long as policy is driven by an obsession with immigration control, any guidance on protection of the vulnerable will be overridden as irrelevant with terrifying ease.

The cross-party parliamentary detention inquiry in 2015, which I chaired when I was a Liberal Democrat MP, called for a time limit on immigration detention and a drastic cut in its use. Since then, there have been countless scandals, revelations and independent reviews. From the detention of Windrush citizens to the abuse at Brook House, the government has responded to the outcry of horror by promising to do better and assuring us that a few tweaks will sort it out.

It is time to face the facts: it is the philosophy that is broken. And it is the most vulnerable people who bear the cost. End this cruel and meaningless practice. There just has to be a better way.

• Sarah Teather is director of the Jesuit Refugee Service UK