Donald Trump said this week that “torture works”, leading to a debate on its effectiveness. I’ve actually experienced the torture he speaks of. In February 2003, at Camp Echo in the US military prison facility at Guantánamo, I signed a confession stating I was a member of al-Qaida. I did it as a result of coercion and torture.

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I’d spent the preceding year at US detention facilities in Kandahar and Bagram, Afghanistan, where I’d been forcibly taken by US forces after being abducted, at gunpoint, from my house in Islamabad, Pakistan, by the CIA and Pakistani intelligence.

On arrival at Kandahar I was beaten to the ground and my clothes sliced off with knives. US soldiers sat on top of me and shackled my bare wrists and ankles so tightly the marks lasted for months. I was then dragged naked, in a bowing position, and photographed, forcibly shaved, punched, kicked, spat upon, and interrogated. After that I was put in a jumpsuit and placed in a cage with automatic rifles pointed at me day and night. The only thing worse than having to endure this treatment was watching, impotently, as it was repeated against other human beings.

It was in this state that British MI5 agents first saw me, hooded, shackled and kneeling. The sight seemed to have troubled them, but did not stop them interrogating me over the next three years.

Weeks later, I was sent to Bagram, where conditions were even more oppressive. As an English-speaker I was interrogated more than most by numerous agencies, including British ones. In May 2002, I was taken into isolation and the CIA threatened to send me to Egypt or Syria if I didn’t cooperate, as they had done to a man called Ibn al-Sheikh al-Libi. They told me he talked within days of arriving in Cairo.

The CIA had me “hog-tied”, with my ankles tied to my wrists from behind and a hood placed over my head, for days at a time. Sleep deprivation, stress positions, beatings and loud noise followed. Bizarrely, the only real respite was the harrowing interrogations. During some of them, I was shown photographs of my children seized from my house and asked where I thought they were now. At the same time, I heard the blood-curdling sounds of a woman screaming from the next room. The effect was devastating. The FBI, who also took part, said they wanted me to confess – to anything.

I next saw their agents soon after arriving in Guantánamo. They’d prepared a document they wanted me to sign, warning me I could face a summary trial and execution if I didn’t. I signed in the hope that the abuse would stop – it certainly receded – and that, once in court, I could expose the reality of what had happened to me. I was eventually released in 2005 without charge or trial and have been campaigning against torture ever since.

During his election campaign Trump said he’d “load up” Guantánamo with more prisoners and reintroduce waterboarding and “a whole lot more”. This week he’s considering the reopening of CIA black sites ordered shut by his predecessor. It’s hard to understand his view, considering the use of torture led to the Iraq war, which he now condemns.

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Under torture, the same Al-Libi the CIA told me about gave false testimony to US interrogators in Egypt that al-Qaida was working with Saddam Hussein on obtaining chemical weapons. Iraq had no al-Qaida presence. But after the invasion the cruelty meted out in US-run prisons such as Abu Ghraib and Camp Bucca became the fuel for the fire of Islamic State; 17 of its top leaders had been tortured in these prisons. Torture did not save lives, it destroyed them and created monsters instead. British journalist and current Isis hostage John Cantlie was waterboarded by his captors, while executioner “Jihadi John” is reported to have waterboarded US national James Foley before decapitating him.

Although President George W Bush blatantly used torture, he never admitted it. He simply got his legal advisers to reinterpret and rebrand it. “Enhanced interrogation techniques” did not amount to torture if they didn’t cause “organ failure or death”, it was claimed. That’s how Guantánamo prisoners such as Abu Zubaydah and Khalid Sheikh Mohammed could be waterboarded hundreds of times without any repercussions for the CIA.

We now know more than in 2002 about what torture does, its devastating effects and its unintended consequences. If the new leader of the “free world” still endorses this barbaric practice, it’s up to the rest of the free world to make sure he has no chance to carry it out.