The smell of gunpowder was so pungent she could almost taste it. Shots rang out; terrified screams filled the air. British-Iranian student Ana Diamond — blindfolded and petrified — believed she had just minutes to live before Iran's Revolutionary Guard opened fire and executed her.

Sentenced to death on the trumped-up charge of spying for the UK, she had served eight months in solitary confinement in Tehran's notorious Evin prison — its youngest inmate — in a tiny cell without a bed or lavatory.

Months of remorseless interrogation had driven her to the brink of madness; even made her doubt her own identity.

Yet still she had not cracked. A student at King's College, London — then just 19 years old — she was visiting her Iranian grandparents for a short holiday when she was captured and imprisoned.

Ana Diamond, 24, has revealed how she was holed up and tortured in the same Iranian prison as British citizen Nazanin Zaghari-Ratcliffe

The Tory activist is a spokesperson for Conservative Future — the Tories' youth wing — and posted photos of herself on social media with luminaries including Boris Johnson

And now she faced death for crimes she had never committed. 'The guards came for me at dawn,' she recalls. 'They drove me from prison, blindfolded, in a van. When they took me out, I collapsed face down and dust filled my mouth. My feet and arms were cuffed. I thought I was walking to my death.

'I remember asking the female officer if she could stay with me. She'd come with me from solitary and, strange though it seems, I'd grown attached to her.

'But she said: 'No. From here, you are with God.'

'I was pushed to my knees. I was trembling, beyond crying. The only thing I could do was breathe. I didn't have a clear thought in my head. I was on my knees, praying; waiting for death.'

Ana's helpless terror was compounded because, in the days before she was told her execution was imminent, she had confessed to a prison warden she was a virgin — a truth later verified by a doctor in a 'demeaning' physical test — because she knew that, under Muslim law, her virginity would save her from the death penalty.

But now she was consumed by dread. 'I was frightened my captors would rape me before they executed me. I wanted to save myself for marriage, and even the idea of rape seemed worse than death. I was told I had one last chance to confess. But I did not want to trade my life for a lie.'

The Iranian-Finnish citizen, pictured shortly after her arrest and detention when she was taken to hospital for a heart condition, was charged with being an MI6 spy

In the prison, food was passed — sometimes thrown — through a hatch in the door. If Ana needed to use the lavatory, she had to call a warder, who slid a piece of cardboard under the door

Ultimately, Ana was not killed. For after a while, her captors told her she was, for the time being, spared. 'He [the executor] does not have time to execute you today,' they said. 'But don't think it's over.'

Ten minutes later she was back in solitary, in her tiny cell.

It is difficult to believe that the young woman sitting with me in a London hotel — composed, beautiful, articulate — has endured such horrors.

One year after her return to the UK, having finally been acquitted of all charges, Ana, 24, is telling her story in full today, exclusively to the Mail, because she believes the world should know the extent of the atrocities perpetuated by the Iranian Revolutionary Guard Corps (IRGC).

'It would be selfish of me, with a voice and a platform, not to stand up and tell my truth. By speaking out I hope to put an end to unwarranted arrests,' she says.

She is talking in a week when reports claim British-Iranian prisoner Nazanin Zaghari-Ratcliffe — sentenced in 2016 to five years in prison in Tehran on a spying charge she strenuously denies — has been shackled to her bed in a psychiatric hospital.

Ana's revelations come, too, as the first photos emerged from inside the British oil tanker captured this month by Iran's Revolutionary Guard.

To understand fully Ana's story of false imprisonment, we need to go back to 2011, when she was 16 and had begun volunteering and campaigning on behalf of the Conservative Party.

Nazanin Zaghari-Ratcliffe, 40, (pictured) was arrested at Tehran's Imam Khomeini airport while travelling with their young daughter in April 2016 and sentenced to five years in prison after being accused of spying, a charge she vehemently denies

On arrival at Evin prison (pictured), Ana refused to sign some documents because she couldn't understand them

Born in Iran, she was raised, an only child, in the UK and Finland by her Muslim father and Christian stepmother.

Officially she has Iranian-Finnish nationality, but she identifies as British-Iranian and has applied for British citizenship.

After boarding school in Helsinki, she came to London to take A-levels and her political activism began. As a spokesperson for Conservative Future — the Tories' youth wing — she posted photos of herself on social media with luminaries including Boris Johnson.

At 18, she began concurrent degrees in film studies and theology at King's College.

Growing up with a mix of faiths, she took up her stepmother's suggestion that she go on a pilgrimage to Israel during the summer vacation of 2014.

She was poised to return to the UK when her mother, in Iran visiting her parents, proposed Ana fly out to see her grandfather for a week before the new university year began. Her father was also visiting Iran on business.

'I was thrilled at the idea of going to see my extended family,' she recalls. 'I hadn't been in Iran since I was a baby and I'd only kept in touch by Facetiming them.'

The first intimations that she was in danger came at Imam Khomeini airport in Tehran, where at a third security checkpoint she was asked by an Iranian Revolutionary Guard where she had come from.

'I said the UK, rather than Israel, which I knew would have been a red flag. They took me into a back room and seized my passport, my laptop and mobile. They questioned me about my family, who are well known in Iran.'

Her father, whom she does not name for his own safety, was editor of the reformist liberal newspaper Salam, which was banned in 1999. She was released and allowed to stay with her mother.

Forbidden to leave the country, weeks became months. 'They asked Dad about my links with the Conservative Party and our espionage work with MI6. Of course I wasn't a spy, but they assumed I was, and thought my dad was the mastermind behind me.'

By now Ana — aware she was trapped in Iran, but determined not to let it curb her academic development — had organised an internship with the UN in the capital. But on her way to work in January 2016, a van pulled up and she was bundled inside.

'The van was packed with a crew of Iranian Revolutionary Guard Corps officers,' she recalls. 'They handcuffed me and forced my head between my knees. That same day I was taken to court. I had red nail polish on and one of the women in the court whispered: 'If you want to help your case, you should start scratching that off.'

Zaghari-Ratcliffe, pictured holding her daughter Gabriella, was arrested on suspicion of espionage

Nail varnish is a sign that someone isn't a practising Muslim. 'I remember madly trying to scrape off the varnish.'

At court, she glimpsed her father, fiercely protesting his innocence, before she was taken to a different room. Then she faced the prosecutor who asked her if she was a spy. 'I said, 'No, obviously not,' and he asked me to write my denial, which I did, in English. This infuriated him. He thought I was trying to provoke him by not writing in Persian. He ripped the papers up.'

She was charged with 52 offences, three of which — espionage, blasphemy and corruption on earth (using her body to gain information from Iranian politicians) — carried the death penalty.

Her nightmare escalated. She was blindfolded, handcuffed and pushed into a car.

On arrival at Evin prison, she refused to sign some documents because she couldn't understand them.

'They said I was being stubborn and it would not help my case. But I refused, regardless.' It is hard to imagine how a teenager remained so courageously single-minded.

But Ana's innocence was the shield that protected her: she was adamant she did not want to suffer imprisonment knowing she'd told a lie.

'They strip-searched me,' she says. 'A female officer felt my breasts roughly.

'Then they started inspecting all my teeth. I later learned they'd been looking for a cyanide capsule, a suicide pill spies sometimes carry inside a tooth.

'Then I had to squat, naked. It was humiliating, but I tried to switch off my mind and pretend it wasn't happening.'

Then she was given a chador [a Muslim full-body cloak] and headscarf to wear, and placed in a tiny cell in solitary confinement.

'The cell was, I'd guess, about six feet by four feet; long enough to lie down in but not to stretch out. The ceiling was about 10ft high and there was a tiny, wired window at the top, from which you could just tell if it was day or night.

'There was a concrete floor — no bed, covers or lavatory — and because it was January, it was freezing cold.'

For five days she saw no one. Food was passed — sometimes thrown — through a hatch in the door. If she needed to use the lavatory, she had to call a warder, who slid a piece of cardboard under the door.

Sometimes she could not summon one in time. 'I'd be banging on the door, crying out with bladder pain until I wet myself,' she recalls.

Both her and her husband, Richard, have staged multiple protests and hunger strikes in attempts to have her conviction overturned

'During my time in solitary confinement I was in a very, very dark place. Your demons come out. You have no connection with reality, no contact with the outside world.

'I was caged there, in a white room, with fluorescent lights on 24 hours a day. Solitary erases your identity. I'd have panic attacks that interrupted my sleep.

'I'd wake with my heart racing so fast I thought it would burst, and for a second I didn't know where I was. It was terrifying.

'But I'm a Shia Muslim and I drew on both the teachings of the Koran and the Bible. Prayer was the only thing that kept me sane.

Perverse though it seems, she came to look forward to the regular, protracted periods of interrogation that allowed her to leave her cell, sometimes for ten hours. 'They'd blindfold me and guide me deep into the prison, down many stairs.'

Ana later learned that both her parents were imprisoned — her father for three years. Both are now free, but, fearing for their safety, she is uncomfortable about giving details of their experiences.

If her interrogators' motives had not been so sinister, there was something almost ludicrous in the absurd questions Ana was repeatedly asked.

'They'd ask me what the Conservative Party was saying about Iran, what their secret plans were. Sometimes they'd scream, break glasses, throw things at my head.

'They questioned me about my trip to Israel, about where I'd gone in Iran — they'd followed my every move before I was arrested.

'One day they introduced me to a handsome Iranian spy, a 'double agent' who worked for MI6 and fed information to the Revolutionary Guard. They asked me if I'd like to work with him. But I suspected it could be a trap.'

When she failed to respond to any psychological tricks, the mental torture ramped up. 'They told me my dad had been executed.

'I didn't know what to believe. I was distraught. They'd try to make me confess by offering me privileges — such as a TV in my cell — in return for information. Once I was put in a cell with another woman, whom they claimed to be Nazanin Zaghari-Ratcliffe.

'They referred to her as 'your British colleague Nazanin'. At the time I had no idea who Nazanin was. I'm not sure it was her, although there was a physical resemblance. We suspected cameras monitored us all the time. We were both worried about saying anything in case it incriminated us. And after five days we were separated.'

Ana reached a nadir when, for 20 days during the Iranian New Year 2016, she saw no one. Then came the awful summons.

'We've come to the point where we know you're not collaborating with us,' they said. 'So we have no option but to carry out your death sentence.'

When Ana protested, they [falsely] claimed that the Foreign Office and her university had been informed that she'd flown back to Britain.

'I realised then that anything could happen. No one would ever know.'

It was at that point, in a desperate attempt to ward off execution, Ana told her captors she was a virgin, knowing Muslim law prevented the killing of young women who had not had sexual intercourse.

She was forced to undergo the humiliating medical inspection that verified this. Then came her fears she might be raped so she could then be killed.

'I started to fear rape more than death,' she says, and her calm composure deserts her. For the first time during our conversation she blots away tears.

In this fragile state of terrified anticipation, she was given a final chance to confess. She declined. 'At that point, all I could do was pray.'

The mock execution was a final, cruel attempt to torture her into making a confession. Yet still she refused.

'And I didn't feel relief when they said they weren't going to kill me.

Fears for Nazanin after Iran debt bid A court ruling over a debt that Britain owes Iran for a military deal has cast a cloud over the release of jailed mother Nazanin Zaghari-Ratcliffe. Judges say the UK does not have to pay £20million in interest on money it owes over the cancelled sale of tanks in the 1970s. Tehran paid £387million for 1,500 Chieftain tanks they never got. Some officials believe Iran sees the money as critical to the release of Mrs Zaghari-Ratcliffe, the Iranian-British dual national imprisoned in Tehran for 'spying'. In the High Court, Mr Justice Phillips said the UK did not owe interest accumulated over ten years on the sum. The Ministry of Defence said it could not comment. Advertisement

'I knew they could still execute me. They could do whatever they wanted and no one would know the details.'

During her captivity she lost track of time, but she believes it was June or July 2016 when she was transferred to a 'public ward' in the prison, where conditions were far less punitive and she had the company of other female political prisoners.

I ask why no one petitioned for her release during her imprisonment.

She says: 'When I was arrested, nobody knew what had happened to me, so no one could advocate for me.'

Finally her mother, freed from prison after two months in solitary confinement, intervened to help Ana.

She and Ana's grandfather convinced the court that, without a passport, she could not escape the country. Bail was set at £100,000, which was put up by her grandfather. Ana was finally released in August 2016. 'But although I was out of prison, the death penalty still hung over me.'

So she did not rest or celebrate: her next task was to appeal against her sentence to try to prove her innocence.

Finally, it was a direct appeal by Ana to Iran's head of legal affairs that secured her return to the UK.

'I went to see him, and he knew absolutely that I wasn't a spy. After long negotiations he helped me organise my acquittal, and when I found out that he'd succeeded, I screamed with happiness.'

By mid-2018, Ana's travel ban had been lifted. She was issued with an emergency passport and fled to Britain. 'I knew I had to go before they changed their minds,' she says.

Two weeks before she left — having lobbied insistently for his release — her father, too, was freed from prison.

It is a measure of her fortitude and resilience that, on returning to London after four years, Ana calmly resumed her studies. She is set to graduate with a first in the New Year.

But although her demeanour is measured and calm, the physical effects have been devastating. 'I still wake at night and my heart is thumping at 270 beats per minute. 'Sometimes I black out. I can't breathe,' she says, showing me the monitor on her chest that charts the wild oscillations of her galloping heart.

She has been diagnosed with arrhythmia and has spent two-and-a-half months in hospital since her return, but two operations so far have failed to cure her abnormal heart rhythms.

Meditation and counselling have helped to restore some of her equilibrium. Outwardly she is cheerful, thoughtful, kind. Inside, resentment — justly — still seethes.

She plans to sue Iran's Revolutionary Guard Corps for the appalling suffering they inflicted on her and her family.

'There are those who might think I should just move on, forgive them as taught by holy texts,' she says.

'But illegal detention — on an international scale — cannot simply be forgiven and forgotten. The IRGC needs to be held to account for the physical, psychological and financial damage they have caused my family over four years. They must answer for their crimes.'