The survey was conducted on brothels in the London area
London's brothel industry has spread to "every corner" of the city, according to a charity's report.
Brothels in the city offer sex for as little as £15, and some are charging £10 extra for unprotected intercourse, the Poppy Project in Southwark found.
Its report said 85% of brothels in the city operated in residential areas and researchers posing as sex buyers found brothels in all 33 London boroughs.
Westminster had the highest number with 71, compared with eight in Southwark.
The study was compiled by the Poppy Project, which provides education about prostitution and helps victims of sex trafficking.
Researchers posing as potential punters telephoned 921 brothels that had advertised in local newspapers.
They found on average 28 brothels in each borough.
Together the brothels generated between £50m and £130m a year, the researchers estimated.
One woman was told she would be working in a restaurant
Campaigners insisted this was just "the tip of the iceberg" as the only source of information was newspaper adverts, as opposed to websites or phone box cards.
Many operated through legitimate businesses - licensed as saunas or massage parlours - though the vast majority were in private flats in residential areas.
The report found 77 different ethnicities among women being offered for sex, many from areas such as eastern Europe and south-east Asia.
The average age of the women was 21. Several places offered "very, very young girls" but did not admit to having underage girls available.
According to the researchers, the average price for full sex was about £62.
Co-author Helen Atkins said: "This research shows the disturbing prevalence of the sex industry in every corner of London - fuelled by the demand for prostitution services.
"Multi-media misrepresentations of commercial sex as a glamorous, easy and fun career choice for girls and women further contribute to the ubiquity of London's brothel industry.
"However, for most women involved in prostitution, the reality is a cycle of violence and coercion, perpetuated by poverty and inequality."
Labour's deputy leader Harriet Harman said there was "a large and growing number" of very young women being trafficked into the UK.
While visiting the project she said she met two women who were brought into the country at the age of 17 "thinking that they were going to come here for a chance for education".
They "were tricked and then forced into prostitution" she said.