By Robin Sidel And Mark Peters 

CHICAGO-- MasterCard Inc. said it has stopped processing ad purchases on Backpage.com, as payment-card networks become the latest front for public officials trying to crack down on the adult section of the controversial classified-ads website.

Backpage has long been under fire from law-enforcement officials, attorneys general and members of Congress who say the website helps fuel the illegal sex industry. And while public pressure helped force its spinoff from Village Voice Media Holdings LLC in 2012, the website has largely prevailed in the courts and public officials argue federal law needs to be changed to curb the sex trade on classified sites.

Cook County Sheriff Thomas J. Dart sent a letter on Monday to MasterCard and Visa Inc., asking them to "immediately cease and desist from allowing your credit cards to be used to place ads on websites like Backpage.com, which we have objectively found to promote prostitution and facilitate online sex trafficking," according to the letter released by the sheriff's office.

"They are being removed as a merchant in our system based on a request from the sheriff's office that we received," a MasterCard spokesman said on Tuesday.

American Express Co. stopped processing ads on the "adult" portion of the site earlier this year, according to people familiar with the matter. A spokesman for Visa didn't immediately respond to a request for comment.

Visa and MasterCard, the two biggest card-processing networks, have latitude to decide if they will process certain types of transactions. Both companies have refused, for example, to allow their cards to be used to buy marijuana in states where it is legal because those purchases are still prohibited by federal law.

Visa and MasterCard will process online-gambling transactions in places where it is legal. But many card issuers don't allow their customers to use plastic for online gaming.

The card networks don't follow the same type of "know-your-customer" rules as banks. Instead, they set rules that are to be followed by the companies that process card transactions on behalf of merchants.

Mr. Dart, whose county is the nation's second-most populous, in the letter to the credit-card companies said allowing such transactions on Backpage provides a sense of normalcy to the sex industry and helps fuel demand for such illegal transactions. The sheriff is expected to release further information on his efforts surrounding credit cards and adult ads Wednesday.

Backpage.com didn't respond to requests for comment.

Pressure has been building over the website in recent years with attorneys general from 21 states calling on the site in 2010 to close its adult-services section. Two years later, Backpage.com was split off from a chain of 13 alternative newspapers, including Village Voice and LA Weekly, as controversy over the classified site grew. The company was then sold late last year to a Dutch company, according to federal officials.

Write to Robin Sidel at robin.sidel@wsj.com and Mark Peters at mark.peters@wsj.com

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