Benjamin Perrin says many of Manitoba's child sex trafficking victims are held in Winnipeg homes controlled by gangs. (CBC)

The majority of Manitoba child victims of sex trafficking are being held captive in "micro-brothels" controlled by gangs, says one legal expert who has written a book on the subject.

University of British Columbia law professor Benjamin Perrin told CBC News Thursday that girls from Winnipeg are routinely trafficked to other cities in western Canada, but many are sold into the thriving underground child sex trade within the city itself.

"Up to 80 per cent of the children in the province of Manitoba being sold for sex acts are being kept inside gang houses in what are called micro-brothels hidden in the community," he said.

Perrin is in Winnipeg to launch his new book, Invisible Chains: Canada's Underground World of Human Trafficking, at a local bookstore. The book is billed as an exposé of human trafficking in the country and a "call to action" to end the practice.

'Craigslist has been called the Wal-Mart of child sex trafficking.'— Benjamin Perrin, UBC law professor

Winnipeg is a prime spot in which to unveil the book given the city's prominence as a hub for child sex trafficking, Perrin suggested.

Winnipeg girls are being covertly sold through websites like Craigslist and then show up hundreds of kilometres away in cities like Calgary, Perrin said.

"Craigslist has been called the Wal-Mart of child sex trafficking," Perrin said.

Perrin called on Manitoba justice officials to demand that Craigslist weed out erotic-services ads that offer sex for money — as officials in Ontario have recently done.

Swan commits to asking Craigslist to remove ads

Three Ontario cabinet ministers signed a Sept. 14 letter to Craigslist CEO Jim Buckmaster applauding the service for agreeing to requests from attorneys general in the U.S. to shut down links to prostitution-related ads.

The Ontario ministers noted the website has not taken similar action in Canadian cities, where there are still hundreds of ads offering sex for money.

Manitoba Attorney General Andrew Swan told CBC News he would follow in Ontario's footsteps and also write a letter demanding the company take the ads down.

Winnipeg police said Thursday they monitor Craigslist ads each day to locate potential victims.

So far in Manitoba, only one person has been charged with human trafficking.

On Sept. 23, Winnipeg investigators came forward with information alleging that a 38-year-old woman confined a younger woman in a North End house and gained control of her property, including her identification documents.

Police said the older woman forced the victim to work as a sex trade worker and kept all of her earnings.

Police also alleged the victim was assaulted and twice prevented from escaping.

Theresa Peebles has been charged with three counts of trafficking in persons, forcible confinement and assault. With file from The Canadian Press and the CBC's Sheila North-Wilson