But consumer advocates say the lack of regulation means that prepaid card users can continue to be blindsided by hidden fees, and have few legal protections to recover their money if a card is lost or a charge disputed.

Mike Henry, who owns a small print shop in California, had not been able to recover $50 stuck on his Only 1 Visa prepaid card after it stopped working. He gave up after numerous calls to customer service  at 95 cents each  went unresolved. Only 1, meanwhile, continued to send daily updates of his balance as it was eaten away by monthly fees. His account was finally whittled to zero.

“For the last six months, I turn on my computer and check my e-mail and get slapped in the face,” he said.

Only 1 officials said they regretted his inconvenience and were refunding Mr. Henry’s money.

Among the beneficiaries of these cards are Visa, MasterCard and Discover, which receive about a nickel to 20 cents each time a credit or debit card emblazoned with their logo is swiped. While the companies do not disclose income from prepaid cards, their efforts to add tens of millions of users represents a potentially significant source of new revenue.

Financial firms that issue the cards are often little-known companies with names like Green Dot, NetSpend and AccountNow. Since they get money upfront from the consumer, there is relatively little risk with prepaid debit cards, compared with credit cards and other loan-making products.

Given the number of people who have little or no relationship with a bank, both in the United States and abroad, the financial industry is betting on a boom. In 2008, for instance, customers loaded about $8.7 billion onto prepaid cards, a 125 percent increase over the previous year, according to the Mercator Advisory Group. The industry is expected to balloon to $119 billion by 2012, Mercator predicts.

“Every year we’ve seen big growth,” said Steve Streit, the founder of Green Dot, now one of the largest reloadable prepaid card companies. “There’s a part of me that believes we are just at the entry ramp to growth right now.”