When Laura Lee, a part-time escort and sex worker rights campaigner, stood up in the Northern Ireland assembly to give evidence against a proposed bill that would make it illegal to buy sex, as in Sweden, she was blunt in her estimation of the damage it would cause. “If the Swedish model is introduced in any way, shape or form in the north or in the south of Ireland, the state will have blood on its hands,” she said earlier this year.

Late on Monday evening in Belfast, the human trafficking and exploitation bill – containing a controversial clause that makes buying sex a criminal offence – was passed with cross-party support, making Northern Ireland the first region of the UK to make the buying of sex illegal, following in the footsteps of Sweden and other Nordic countries such as Norway and Iceland.

Lee fears that the change will increase the danger for prostitutes. “This will increase violence, make sex workers less likely to report crimes to the police, increase stigma and force the sex industry underground,” said Lee. “I find it incredible that people push this moralising crusade in the face of all the evidence which says the decriminalisation of the sex industry is the only way to improve the welfare of sex workers.”

But there are a growing number who want a similar law in England. Emboldened by the vote in Northern Ireland, a coalition of campaigners including women’s groups, MPs, unions and former sex workers, this week launched the End Demand campaign, and announced that Labour backbencher Fiona Mactaggart was set to propose an amendment to the modern slavery bill later this month. Currently buying and selling sex is not illegal elsewhere in the UK but soliciting, pimping, brothel-keeping and kerb-crawling are all criminal activities.

The chances of the Mactaggart amendment making it into the bill may yet be slim, but the MP believes that the vote over the water is a game changer. “We now have part of the UK with a sex buyer law – now we have to make sure we can have the same for the rest of the country,” she said, at the launch event in Westminster, arguing that it was necessary to have parity across the country.

Dublin-based charity Ruhama, which has provided services for prostitutes since 1989, said a law criminalising the buying of sex would have a significant deterrent effect. “We see the harm done by prostitution and we know that the profits of prostitution do not go to the women working, but to criminal gangs,” said Gerardine Rowley.

Meanwhile, some figures in Northern Ireland remain unpersuaded. Justice minister David Ford argued a change in the law would drive the sex industry underground and put its workers at greater risk. Research commissioned by the justice department – which estimated that the country had 20 street sex workers and around 300 working off-street – found that only 2% of women and men working in the local sex industry were in favour of the Swedish model, 61% thought it would make them less safe and 85% said it would not reduce sex trafficking. The law will decriminalise street sex workers, who will no longer be charged for soliciting, but it will still be illegal for two women to work together, or to run a brothel.

The proposed change has also left some sex workers in Belfast worried about their income. Hannah, an independent escort who works from her own premises, told the Guardian that far from being a vulnerable woman who had been forced into prostitution, she had begun the work because it provided a good work-life balance. She said when the decriminalisation clause was passed she “cried and cried – the city I love is taking away my means of earning,” she said. “I don’t want this, I have the abilities to work, why are they taking my job away?”

Former detective superintendent Alan Caton, who led Ipswich’s response to the murders of five women who worked as prostitutes in 2006, emphasised a different approach, that police had successfully operated a “zero tolerance” approach to sex work after the murders – cracking down on kerb crawlers, while not prosecuting women. Significant funding which helped women with housing, benefits and healthcare enabled them to leave the sex industry. Prior to 2006 there were 107 women working on the street in the area, but he said there were now none.

There is no accurate data on the number of prostitutes in the UK. A commonly quoted figured of 80,000 dates back to 1999, a piece of research from the UK Network of Sex Work Projects.

“It’s about tackling demand, and from my experience that can work.” Jonas Henriksson, an inspector from the Stockholm police, told the End Demand event he was “a simple cop, trying to arrest as many sex buyers as possible” and explained that Stockholm police now have a six-person unit dedicated to tracking sex buyers and they arrested between one and four buyers on every shift. Most men admitted their guilt immediately, he said. “Buying sex in Sweden is a shameful crime, people do not want to go through a trial.”

Sweden introduced a sex buyer law in 1999, and was followed by Norway and Iceland. Laws are currently being debated in France and Canada, while the European parliament voted to push the Nordic model, which is also supported by the UK all-party parliamentary group on prostitution.

According to 2010 research from the Swedish government the law has halved street prostitution, while the number of men paying for sex declined from 12.7% in 1996 to 7.6% in 2008. The law also changed society’s opinion about buying sex: in 1996, 45% of women and 20% of men supported criminalising the buying of sex, but by 2008 numbers had grown to 79% and 60% respectively.

But sex worker rights groups argue that the Nordic model has been a failed experiment and has increased stigma, and point to a UN HIV Law Commission report [PDF] which states that “since its enactment in 1999, the law has not improved – indeed, it has worsened – the lives of sex workers”, adding that while street work has halved the sex trade remains at pre-law levels.

A few miles away from the halls of Westminster in a flat in Soho, Ana, a sex worker from Romania, is fiercely opposed to the End Demand campaign. Taking a break between clients in the flat’s kitchenette she says she took the work by choice, because a friend told her about the money that could be made.

She has been working in Soho as a prostitute for six years, since she was 20. “I’m 26, who’s going to tell me what to do? I’m here for the money, this is my choice”. She is saving up to buy a house, and says her dream is to “get married and have 10 kids.” She admits that clients are sometimes violent and says “it is not easy to do this job”, but says girls would be less safe if their clients were criminalised. “It is none of their business, they should just leave us alone.”

Some names have been changed.