A row of small - and seemingly respectable - terraced houses opposite a school in East Croydon gives no obvious clue to what is going on behind closed doors.

The only thing which marks out one of the houses from the others is a tiny camera positioned outside its front door.

This particular establishment is one which has been monitored for some time by Croydon Community Against Trafficking (CCAT), which believes it is a brothel using trafficked women. The camera, it claims, is there for self-preservation.

"I wonder if the neighbours know what they are living next door to," a member of CCAT who did not want to be identified said. "These places are operating all over the borough - in some places you wouldn't even suspect."

According to CCAT, sex is being sold metres away from where schoolchildren are at play and also close to gyms, public transport - and even near the borough's law enforcers.

Just last week, the Croydon Guardian revealed how officers had raided a brothel 500m from Croydon police station, acting on information gathered by CCAT.

A woman who was thought to have been trafficked from Estonia has since been taken home.

She was too scared to become an official police witness and trafficking charges could not be brought against the proprietor.

CCAT - a voluntary organisation made up of residents, businesses and other concerned individuals - was set up after it was revealed Croydon has one of the largest sex industries in the country.

The borough was also identified as one of the towns where trafficked women were being sent in significant numbers, according to a report published by the London-based Poppy Project.

Since the publication of the report, CCAT has been monitoring the borough's 100 or so brothels.

Male members from the group regularly ring round escort agencies and massage parlours to determine what kind of "services" they offer.

The evidence is then passed on to police.

CCAT's research is being taken so seriously police have used it to raid more than 20 brothels in two years.

"Most of the places we call seem to have access to a range of girls of all different kinds of nationalities," a CCAT spokesman said.

"The main clues is the range of girls, the cost of the specialist services they offer and the fact they offer sex without condoms.

"Brothels will often say they have a nice Japanese girl' or an Italian woman. These are almost certainly women from poorer countries in that region, such as Burma or Armenia.

"When you think of it logically, why would a woman from a prosperous country like Japan or Italy come to work here in a brothel, offering sex at rock-bottom prices?

"You have to ask how a woman from a poor country could afford to come here. It's very likely she's been trafficked."

Another CCAT member revealed how anonymity was vital in their line of work.

"It can be a bit nerve-racking," another male member of CCAT said. "You're potentially dealing with some very nasty people.

"We keep our identity hidden to protect ourselves from retaliation and to allow us to carry out our research without arousing suspicion.

"Ironically, being a bit nervous actually works in your favour. I guess a lot of guys who genuinely use brothels must behave like that."

The Croydon Guardian was taken to about 10 establishments all over the borough - stretching from the north to the leafy south - during one morning last week.

Many of the brothel's receptionists used code words to explain what kind of extras their girls offered.

However, we were surprised to hear one business offering sex in plain terms.

While sitting outside an unassuming terraced house close to West Croydon station, a CCAT representative called the massage parlour's phone number and put them on speakerphone.

An Oriental woman - clearly not yet au fait with the code - reeled off a list of girls, their nationalities and the type of sex on offer, some of which cost as little as £50.

She then gave out the address and offered directions to make the punter's visit as easy as possible.

Securing trafficking charges is a difficult task for police. Even if they successfully raid a brothel they need proof of trafficking, which is not easy if women are too scared to give evidence against their captors.

However, the Met Police are continuing to tackle the issue through their specialist trafficking and sexual exploitation unit Operation Pentameter.

A CCAT spokesman added: "It's a hard battle and not one that's going to be solved easily. We just need the community to be aware of what's going on.

"National statistics show that 80 per cent of brothels in the UK use trafficked women. If you apply that to Croydon, a significant number of brothels here are highly likely to be doing the same.

"Trafficking is a form of modern slavery. We would like to see the law changed so that anyone caught having sex with a trafficked woman is charged with rape. After all, if she's being forced into prostitution she's not giving her consent, is she?"