In December last year religious leaders from around the world gathered in Rome to sign the Global Freedom Declaration on modern day slavery.

This is a call to action to eradicate modern slavery across the world. Churches in Scotland have spent many years championing Scottish work to eliminate human trafficking and raise awareness of this injustice being perpetrated in our own communities as well as around the world.

The Scottish Parliament is currently considering legislation designed to help the victims of trafficking and to stop the criminals who profit from slavery. The Human Trafficking and Exploitation (Scotland) Bill is very welcome and the Scottish Government and all political parties should be encouraged to scrutinise it in a constructive way.

An important aspect of trafficking, on which many church and faith groups have called for action, is that of sex slavery. The majority of people trafficked to the UK are women and children, and sexual exploitation is the most prevalent form of slavery to which trafficked people are subjected. It is a question of both global and gender inequality. Forced sexual exploitation is an uncomfortable subject to think about, but it is happening now, and there are victims here, today, in Scotland.

It is the demand for sexual services that makes bringing women and girls to Scotland profitable for traffickers. Sex trafficking is exploitative, is characterised by violence, and is harmful to women and the whole of society. We are asking the Government, political parties and all those with an interest to take part in a serious debate about how best to tackle trafficking for sexual services. We believe that what is needed is a change in the law to criminalise the purchase of sex and that the Bill at Holyrood provides the ideal platform on which to debate these issues.

We are calling for the Scottish Government to follow policies which have been agreed at a European level, including a vote by the European Parliament in 2014 which affirmed that criminalising the purchase of sexual services would be a way of "combating the trafficking of women and under-age females for sexual exploitation and improving gender equality".

This has been law in Sweden since 1999 and has been adopted by several other countries and legislatures since then. Late last year the Northern Ireland Assembly voted to introduce this law, which is sometimes known as the Sex Buyer Law. There is evidence to show that where it has been introduced, demand for prostitutes has fallen and there has been a corresponding fall in the number of women and girls trafficked for sexual purposes. In countries which have legalised prostitution, such as in Germany, there are many more people trafficked. In 2004 Germany had 62 times as many people trafficked for sexual exploitation as Sweden, despite having an overall population only 10 times bigger.

Prostitution and sex trafficking are inextricably linked. Critics of the Sex Buyer Law point to prostitution being pushed underground, rendering sex workers more vulnerable to violence. We recognise that violence is inherent in prostitution, irrespective of the law, and are concerned for any sex worker in such situations. However, evidence clearly points to reduced demand for commercial sexual services following the introduction of the Sex Buyer Law. The increasing use of technology such as websites, blogs, social networking and mobile phones to advertise sexual services provide tangible means for the police to investigate and collect evidence.

We know that the issue is complicated and it needs careful thought; what Scotland needs is a proper debate based on evidence. Balancing the risks associated with criminalising the purchase of sex is undoubtedly complex. We shall be making these points to Michael Matheson MSP, the Cabinet Secretary for Justice, when, along with members of other denominations and faith groups, we meet with him on Wednesday 29th April. We shall also be highlighting the need to address the life circumstances that lead women to work in the commercial sex industry and to resource routes out of prostitution for those who do.

Whether you support the Sex Buyer Law or not, we need to speak out with one voice calling for modern slavery to be eradicated.

Professor Watson is convener of the Scottish Churches Anti-Trafficking Group.