GROUNDBREAKING police work has revealed nearly 3,000 women are working in Scotland's booming off-street sex industry.

The new single force has carried out its first nationwide headcount of the vice trade and discovered there were as many indoor sex workers as shipbuilders north of the Border.

The scoping exercise comes as senior officers say they are determined not to criminalise individual prostitutes but are desperate to gauge the scale of an industry dramatically transformed by internet advertising and the rise of porn culture. One source said: "Our priority is keeping women safe, whether they are on-street or off."

A snapshot as recently as last week found 2,253 individuals advertising sex for sale on a series of escort and other websites, including a major single portal hosted in south-east Asia. However, sources stressed numbers have been close to 3,000 in recent months as the market - largely featuring foreign women who move around or are moved around - ebbed and flowed.

The sex trade has moved off the streets in recent years as women working in flats replaced traditional streetwalkers, most of whom were Scots with addiction or debt problems. Police sources said they had identified women speaking 20 different languages, with many from eastern Europe, Latin America and south-east Asia.

Detective Chief Inspector Ruth Gilfillan, of the National Rape Taskforce and Human Trafficking Unit, said she believed "well over 90 per cent" of sex work was now carried out from flats or brothels - premises with more than one individual having sex for money. She said: "Off-street reflects the modern picture of what prostitution is. In the past a lot of the research has been on on-street when in fact the off-street is the one where there are the most significant numbers.

"I do think it's important to emphasise the risk to those engaged in on-street will remain. But the challenge now is for us to engage with women working off-street."

Police and other agencies have long focused on the safety of women working in red-light areas. However, sources admit they have traditionally remained unaware of the scale of indoor sex trade and the dangers posed to "invisible" sex workers, including foreigners.

Ms Gilfillan stressed safety was her concern. She is looking to extend the "Ugly Mugs" scheme - an English-based system under which sex workers can check the telephone numbers of potential customers to see if they have caused trouble before - north of the Border.

"It is important those involved in the off-street sex industry understand our position is not to criminalise them. There are those who are being forced or have no alternative but to work in the off-street industry. And there are those who have made an informed lifestyle choice. Some people are content with this choice. But there are also people who are not.

"Our priorities are to identify people and organised crime gangs who are criminally exploiting those people."

Selling sex is not illegal in Scotland, provided a prostitute is working on his or her own indoors and does not solicit outdoors.

Selling sex in premises with more than one prostitute, such as in saunas, is illegal - and anyone organising this could be charged with brothel-keeping or living off immoral earnings.