Researchers from the Poppy Project, which campaigns against the trafficking of women, posed as clients looking for sex and telephoned adverts listed in local newspapers.

They found 921 operational brothels across the capital, offering full sex for between 15 and 250 pounds.

They estimated that at least 1,933 women aged between 18 and 55 were working in the establishments, an average of two per brothel.

The charity said the full scale of prostitution was likely to be much larger as the survey did not cover adverts placed on cards in telephone boxes or on websites.

But it estimated that between 50 and 130 million pounds a year was being earned by the brothels identified through newspaper adverts.

When women personally providing sexual services answered the phone they regularly sounded miserable, subdued, tired or exhausted, the researchers reported.

"For most women involved in prostitution, the reality is a cycle of violence and coercion, perpetuated by poverty and inequality," said Helen Atkins, co-author of a report based on the charity's survey.

Many of the brothels were running as legitimate businesses, licensed as saunas or under the guise of massage parlours, although the majority were private flats in residential areas.

Westminster in the heart of the capital had 77 brothels, more than any other London borough, while Southwark, on the south side of the Thames, had just eight.

The survey found prostitution spread far beyond the inner city, with three-fifths of the premises being located in outer suburban boroughs.

Government research published in January found that 75 percent of regional and local papers carried adverts offering women for sex.

Since then the Newspaper Society has tightened up guidance leading to a number of newspapers refusing such adverts.

But Minister for Women Harriet Harman said the Poppy Project report showed that more newspapers papers needed to follow suit and stop fuelling demand.