AMSTERDAM – Mobile phone users around the world will spend $1 billion a year on pornography sent to their handsets by 2008, which may boost the wireless services sector much as it fueled growth in the fixed-line Internet, a market research firm said.

In the United States, consumers will be dishing out some $90 million for adult entertainment in four years’ time, the Yankee group said in a survey released on Monday.

Mobile operators, which are struggling to drive up data traffic as a new source of revenue, have so far been reluctant to cash in on the opportunity, although many industry experts consider pornography to have been the first profitable service offered on the fixed-line Internet in the second half the 1990s.

Excluding portals of U.S.-based mobile operators, which fear possible repercussions if pornographic content becomes accessible to children, half of all wireless data traffic consists of adult content, Yankee said.

“Fear is trumping greed for the moment, but the two can work together if carriers can develop a solid mechanism for protecting minors and safely profit from the opportunity,” Yankee said.

The British unit of Vodafone, the world’s biggest mobile operator, has put child protection measures in place.

PhoneErotica.com, controlled by wireless startup PhoneBox Entertainment, receives more than 75 million hits to its Web site per week, Yankee said. Most of the users only pay the operator for the air time, since the service is initially free.

The entertainment company hopes to get mobile carriers to let it piggy-back onto their billing systems, due to difficulties with credit card payments, Yankee said.

Credit card companies demand high tariffs from porn sites, reflecting the large volume of bogus card numbers and other credit card abuses that often plague those sites.

Research by PhoneBox has also found that only 5 percent of customers were willing to send their credit card numbers over a Web-connected phone.