Craigslist, the world's most popular online classified advertisements site, is being sued by an Illinois sheriff who alleges that the site is a clearing house for prostitution.

Cook County Sheriff Thomas Dart in Chicago has filed a federal lawsuit calling for a ban on the Erotic Services section of the website. He claimed that the owners of the site facilitated prostitution by failing to block blatant offers to trade sex for money.

"Craigslist is the single largest source of prostitution in the nation," Dart said at a press conference in Chicago. "Missing children, runaways, abused women and women trafficked in from foreign countries are routinely forced to have sex with strangers because they're being pimped on Craigslist."

The website, set up by Craig Newmark in 1995 after he started e-mailing friends with lists of local events in San Francisco, receives more than 500 million visitors a month. It runs classified ads, mostly free, for 450 cities or regions in 50 countries around the world, including nearly 30 in Britain.

Dart filed his lawsuit four months after the company settled a nationwide lawsuit by promising to enact new rules to crack down on prostitution.

"This section is a convenient clearing house for pimps, prostitutes and patrons that enables sellers to advertise and buyers to peruse discreetly," Dart said in court filings.

Dart said that he had made numerous written pleas to Craigslist asking it to close the Erotic Services section. In the past two years his department has made more than 200 arrests linked to the website on charges that include juvenile pimping, human trafficking and endangerment of a child. The people arrested have ranged from heroin addicts to suburban soccer moms, a former reality TV star and teenagers as young as 14.

Dart asked a federal judge to order Craigslist to remove its Erotic Services section. He acknowledged that the company did warn that solicitation of prostitution was prohibited but said that it ignored obviously illegal ads. "None of these ads require any imagination. There's no mystery," Dart said, pointing to ads with phrases such as "Teens for cash ... $100 quickie," and "Ask me about my 2 girl specials."

He is also seeking reimbursement for money spent on the salaries of officers who investigate and arrest those responsible for trafficking prostitutes on the website.

Craigslist said in a statement that it co-operated with the authorities and it was unwise for users to promote illegal activity via the site. "Misuse of the site is exceptionally rare compared to how much the site is used for legal purposes. Regardless, any misuse of the site is not tolerated on Craigslist."

Dart said an FBI investigation found last year that more than 2,800 child prostitution ads had been posted on Craigslist and a recent nationwide sweep for child trafficking and prostitution led to hundreds of arrests. "Pimps are preying on the most vulnerable members of our society and taking advantage of our struggling economy," Dart told reporters.

"The worst part is Craigslist's owners know their website is still being used for illegal purposes and they're doing nothing to stop it."

The company has said that its site is "community-moderated," and it relies on users to flag problematic postings. It has a staff of only 28 and, with more than 30 million new classified ads posted every month, it is impossible to monitor and review each individual ad.

However, last November, in a settlement with attorneys general from 40 states, Craigslist agreed to crack down on "erotic services" ads, requiring anyone who places one to give a working contact number and pay $5 via credit card, which is given to charity. If subpoenaed, the site hands that information over to law enforcement agencies. The move has reduced postings to the Erotic Services section by as much as 40 per cent, according to some reports.

But there are still hundreds of new "erotic" ads posted every day in every city in the US. Dart claims that the traffic Craigslist generates from prostitution accounts for the bulk of the website's popularity and has allowed it to generate $80 million in revenue last year from paid posts for jobs and accommodation.

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