Sairah Vanar has been married for four years, but has never slept in the same bed as her husband, Kabir. She keeps a lock on her bedroom door. If she bumps into Kabir in the hall when she comes home from university, she says hello politely, as she would to any passing acquaintance. Kabir says he loves her, but Sairah thinks that's just misguided Asian notions of pride and honour talking. She is tiny, Sairah, like a child almost, big dark eyes flashing behind her glasses. Sometimes, in those moments in the hall, Kabir prolongs the conversation, implies he wants to sleep with her, and she gets nervous. But a mental strength has developed beneath Sairah's physical fragility. She gets herself some food, goes to her room to study, and slips the bolt over. "If I did not have hat lock, I would be so scared," she admits. "I am a tiny little thing and he is this big tall guy."

Sairah's parents say her marriage was arranged. She says it was forced. In October last year, David Cameron called for the contentious issue of forced marriage to be the subject of a public consultation, which will conclude at the end of this month. The consultation will decide whether forced marriage should become a criminal offence, punishable by imprisonment. Describing it as "little more than forced slavery" Cameron said we should not shy away from addressing the issue because of "cultural concerns".

Currently, potential victims of forced marriage are protected by civil law. They – or their representatives, such as teachers or social workers – can take out a forced marriage protection order (FMPO). The order lays down conditions that try to change the behaviour of anyone trying to force a person into marriage. But FMPOs are hard to monitor, and there is no automatic sanction for breaking them. Last November, Scotland became the first country in the UK to make breaching the orders a criminal offence and Cameron has pledged to do the same for England, Wales and Northern Ireland. But he has gone further, asking the home secretary to consider making the very acto of forced marriage a criminal offence.

Criminalisation has been considered several times over the years but rejected because of fears it would lead to a drop in reporting. "Criminalising forced marriage would push it under the carpet," argues Sameem Ali, a Labour councillor for Manchester's Moss Side, who was once a victim of forced marriage herself. "If we criminalise, we can forget about the victims. They will not come forward."

Ali was taken to Pakistan from Britain and forced to marry at 13; at 14, she was a mother. She gave birth here, yet not one professional questioned her plight, or considered child abuse. Wouldn't a law raise awareness? No, says Ali. Asian girls are too reluctant to publicly oppose their own families. Ali was put in a children's home at six months old but brought back to the family home at the age of seven. Her mother beat her, verbally abused her and made her the family's domestic servant. Yet Ali says she would never have resorted to legal action. Why? "Because I didn't hate her. I still cared about her."

Jasvinder Sanghera disagrees with Ali. Sanghera fled her own family nearly 30 years ago to avoid a forced marriage. Years later, her sister killed herself by setting herself alight to escape an unhappy forced marriage. She wants the law changed. Sanghera runs Karma Nirvana, one of the few British charities dealing with forced marriage, and its Home Office-funded helpline receives 500 calls a month – 12% of which are now from men. The government's Forced Marriage Unit also deals with around 1,500 cases a year, but it is thought many more go unreported. "Victims are saying we need the full protection of the law," argues Sanghera.

Karma Nirvana has conducted its own research as part of the public consultation process. So far, it has had 1,620 responses: all, bar a couple of neutral responses, support criminalisation. It's important, says Sanghera, that society's message is clear. "We're trying to create a cultural responsibility here. It's our duty to bring this above ground. If we look at countries where this is criminalised, such as Denmark and Germany, there has not been a decrease in reporting. Unless people can show me there will be a drop, I want to see it criminalised. This is an offence that is not to be tolerated, an offence that can – and does – end in violence, rape and murder."

Sairah, the woman who sleeps with a lock on her door, was 15 when her parents asked if she wanted to get engaged to a 16-year-old cousin Kabir in Pakistan. She thought they were joking. She had moved from England to a small Scottish village and they had heard about her crush on a white boy in school. The two never even went out together, but her parents were horrified. For Sairah, there was always a conflict between her life as a British school girl and her life as a "good" Asian daughter who did not go out in mixed company without a male escort. Occasionally she had screaming matches with her mother about her lack of freedom but, ultimately, she accepted the restrictions because it was the same for all the girls in her extended family.

White, western culture has had its own struggles accepting women's sexual autonomy. But along with upholding family honour, controlling female sexuality is a common motivation for forced marriages. Once a white boyfriend became a possibility, Sairah was told her family's honour rested on her engagement. To buy time, she agreed to consider it, but that was interpreted as consent and a flow of congratulatory calls from family ensued. Her parents calmed her protests. Nothing was settled; they would go to Pakistan to meet her cousin. At school, her grades dropped. At home, she was showered with gifts and money. It was like being a mini celebrity.

"I was attracted to the attention," Sairah admits now, "I feel so stupid when I think about it."

She spent her 15th birthday in Pakistan and what she thought was her birthday party turned into her engagement party. Her cousin Kabir who had been oblivious to the wedding plans, simply accepted it when he was told.

Back home, Sairah tried repeatedly to break off the engagement. "At times, I'd think: 'I can't let this happen.' At others, I'd think: 'They are your mum and dad, for God's sake. They are not going to make any wrong decisions for you.'" There were always tears when she said she couldn't get married. Sairah had never seen her father cry, even when his brother passed away. He did not cry in front of her, but she would see him red-eyed. "To me, he was such a strong person. But I had caused this. I had made my dad cry. That broke my heart."

David Cameron was at pains to make a distinction between arranged and forced marriage. "The government has given us a clear-cut definition of what the difference is," says Jasvinder Sanghera, "but what we know from victims is that, actually, the line is sometimes blurred." John Fotheringham, a child and family law specialist who was part of the Scottish government's consultation process on forced marriage, agrees. "The situation where someone says, 'Marry this man or you will be killed,' is very extreme. People don't say that often. What they say is, 'Marry this man or your mother will kill herself and your university funding will be cut off.'"

Once, domestic violence was seen as something private. It was a family's business, not to be interfered in. It took time for society to recognise that living with a person did not give you ownership of them. There has been a similar reluctance to interfere in forced marriage, partly, Sanghera points out, because people are frightened of being deemed racist for not respecting another culture. But forced marriage is not a cultural matter. It is internationally recognised as a human-rights issue. It is also outlawed by every major world religion, including Islam.

On her wedding morning in Pakistan, Sairah's hands were being painted with henna designs, when she passed out with the stress. "I knew inside myself that I was not going to be his wife because I did not love him." Asian brides traditionally marry in red, bedecked with heavy gold jewellery. Sairah wore delicate pistachio green with a thin chain around her neck. This was her rebellion: at 18, it was the only kind she was strong enough for. "I kept my gob shut," she says.

Reluctantly, she admits to several suicide attempts from the age of 15. "I wouldn't do it now," she says quickly. But when she talks of the despair of those attempts, it becomes clear that forced marriage is an intractable problem because it's not just about coercion. It's about love, and sometimes love becomes entangled in strange notions of control, entitlement and even cruelty. Suicide was preferable to disobedience for Sairah. And her parents were similarly conflicted. Last year, after Sairah's last suicide attempt, her mother brought her home to look after her. When she woke from a deep sleep, Sairah felt a presence. She didn't move. Her father was lying beside her, gently stroking her head. "As soon as I showed I was awake, he walked out of the room," she recalls. "He didn't want to show me he cared."

Sairah's story is poignant because it has elements we can all recognise: complicated family dynamics, a child's need for approval even when they have clashed with parents; the often stubborn belief that parents know what's best for their children and that the end therefore justifies the means. But sometimes the "means" involves violence.

There is something about 20-year-old Sahrish Rizvi that alerts you to trauma. She is simultaneously vulnerable and demanding, emotional and mutinous, some days feeling like talking and others not. At times, she seems overwhelmed. Taken from Warrington to Pakistan at 17, she says she was locked alone in a room, beaten by her own father and told she would marry a man of his choosing, not Abbas, the man she loved. Two years later, Sahrish still feels damaged by events. "I changed as a person," she says, so quietly she is almost impossible to hear. "It got inside my head. I got ill, lost weight. Although Abbas is now my husband, I have other conflicts. I never got married as an Asian girl should."

Abbas was studying business and administration, sponsored by the Pakistani government, when they first met. Sahrish's father was furious. Abbas was a Shia Muslim, and he was 29; Sahrish was 17 and a Sunni Muslim. Abbas tried phoning her father to reassure him that he loved his daughter. He says that her father threatened to break his legs. Sahrish both loved and feared her father, but she defied him. "Once I met Abbas, I wasn't scared of anyone any more."

When the couple sought police protection, she says, her father's attitude changed. It was all a misunderstanding: Sahrish could marry Abbas. She had always dreamed of going to Dubai to buy her wedding dress and, soon after, her father took her. But then he insisted they go on to Pakistan. Once there, Sahrish says, he told her to obey him or she'd never return to the UK. She called Abbas before her phone was confiscated. "I am in trouble," she said.

Abbas is from a family of senior army and political figures and, with their help, he traced Sahrish's address. In Britain, he contacted the police, who were investigating the suspected honour killing of Shafilea Ahmed in Warrington, and urged caution. Alarms were fitted in his Manchester home and he was warned not to go out unless necessary. "But to be honest, I was planning to buy my ticket and go to Kashmir to save her because I had the feeling she might be killed. When you fall in love everything changes. I fought for her because I was blind in love."

Back in Pakistan, Sahrish watched her father being kidnapped by armed men who said he could not allow his daughter to marry Abbas because he was already married. Sahrish was confused: she did not believe Abbas was married. Later, she came to suspect that her father had staged the kidnap. On his "release" he brought a photograph back, supposedly showing Abbas with his wife. Sahrish recognised the photograph and knew it had been doctored. She said nothing, and promised not to see Abbas again.

Armed with information from Abbas, officials raided Sahrish's Pakistani address. Her father was out, but he knew they'd be back. Reassured by Sahrish's false promises, they returned to the UK. Her father was arrested on arrival and Sahrish was taken into care, unable to see her family or Abbas.

Soon after her 18th birthday, Sahrish married Abbas, a rushed Mosque ceremony without family or friends. Abbas decided that they did not want Sahrish's father prosecuted. "In my culture, in my religion, in my customs, the wife's father is equal to a real father. So I paid him respect. And I never got it back. He still hates me."

He and Sahrish still struggle with that in their relationship. Abbas recognises the pull, the power, of Asian family ties. "It has affected everything. I feel they may take her back." He shakes his head. "You don't understand Asian people. If they want to be bad they are worse than anyone. If they want to be good they are like angels."

More needs to be done to prevent forced marriage, Abbas continues. "This is a very big offence. An offence where you are killing someone, indirectly, inside. It's not physical, it's internal. It makes you mad. Makes you crazy. Makes you disabled."

Laws already exist that deal with the various elements of forced marriage: kidnap, rape, assault, child abuse. Councillor Sameem Ali believes that legislation is sufficient. Her own brother was sentenced to four years' imprisonment for conspiracy to kidnap after police intercepted a group carrying weapons and a piece of paper with Ali's address when she fled her marriage. But Jasvinder Sanghera says a separate law might give women more confidence to escape their plight. Nobody would be forced to use this law; what is there to lose? "Let's be clear, this isn't about making people take anyone to court," insists Sanghera. "I don't want to see lots of people thrown in prison. People will still be able to go down the civil route if they want."

Changing the law may send out an important message, but it won't be an instant solution. Sahrish and Abbas did not prosecute Sahrish's father. Sairah locks her door rather than seeking legal annulment. And that is at the heart of this dilemma. There are few crimes in which the victim usually wants a continued relationship with the perpetrator. What Sanghera and Ali agree on is that education is crucial. Most perpetrators do not recognise their own behaviour as wrong. Sairah once told her mother she did not sleep with Kabir. Rather than blaming herself for her daughter's sad situation, her mother screamed and wept and said it was shameful. "She thought I was the devil and it was one of the most outrageous sins ever."

Sairah is now in a secret, serious relationship with a man called Salim. They want to marry, a situation that puts them both in danger. Her uncle found out and threatened to beat Salim up so she has to pretend they are no longer friends. On the surface, nothing stops her leaving Kabir to be with Salim. She will do it when she finishes her course, she says. When she is stronger. But the real hesitation – the hesitation of every victim – is that it will split her family. She will keep her sisters but lose her father. Wistfully, she says maybe she will still see her mum – behind her dad's back. Right now, nothing can solve her problem. So for the moment, she bolts her door and waits.

Some names have been changed