A change in legislation four years ago, which made buying sex a criminal offence, has had a ‘minimal effect’, according to a new report.
The Department of Justice has published an independent review into the offence of purchasing sexual services.
The move in Northern Ireland in 2015 echoed laws already in place in Scandinavian countries that opt to make customers guilty of crimes, rather than the prostitutes.
The latest research published on Thursday was carried out by Queen's University Belfast, and looks at the impact of the legislation on the demand for sexual services, the safety and well-being of sex workers, and human trafficking.
The review reported that while on-street prostitution has declined, the number of sex workers advertising online increased.
It estimated the number of sex workers in Northern Ireland rose from 3,351 up to 3,973.
An increase in demand for sexual services was also reported by sex workers in the period following the introduction of the legislation.
On-street prostitution declined in comparison to previous research, reducing from an estimate of 20 active on-street sex workers operating in Northern Ireland in 2014 to currently less than 10.
In the period from June 2015 to December 2018, there had been 15 arrests and two convictions for purchasing sex and 31 arrests and two convictions for human trafficking for sexual exploitation.
The research found it was "not possible" to conclude that the law was responsible for any increase in crime against sex workers
However, sex workers reported that a heightened fear of crime had contributed to feelings of marginalisation and stigmatism.
The review concluded that the legislation has had minimal effect on the demand for sexual services; and due to the absence of any evidence that demand had decreased, it was ‘unable to determine how the offence could have impacted on human trafficking.’
Kate McGrew, spokeswoman for the Sex Workers Alliance Ireland (SWAI), said the research only highlighted an increase in the sale of sex and the health of sex workers put at risk.
"If the purpose of the law was to decrease demand, it has failed," she said.
"If the purpose of the law was to help sex workers, it has failed."
She added: "This was predicted by sex workers, who were ignored during the process of introducing the laws.”
The Nordic Model legislation was introduced by the Stormont Assembly in June 2015.
This model criminalises the purchase of sex, not the sale, and supporters argue it helps support women who have been coerced into prostitution or sold into trafficking.