Millions of wireless customers are planning to switch to prepaid service, a new study conducted by Infogroup/ORC has found.

According to the company, which conducted its study for the New Millennium Research Council, 20 percent of wireless customers (estimated to be about 24.6 million adults in the U.S.) will move to "less expensive unlimited prepaid wireless service with no early-cancellation penalty" at some point within the next six months. An additional 10 percent of respondents said they would switch to a prepaid plan if they weren't "subject to an early-cancellation penalty."

The vast majority of those who don't want to switch from current plans--61 percent of that group--said that they "don't want to lose [their current phone] and its features." Cancellation charges was the second most popular reason given for why respondents wouldn't switch to a prepaid offering.

Infogroup found that 59 percent of folks who are currently under contract with a carrier would be "very likely" or "somewhat likely" to switch to a "no-contract or prepaid phone" if they were offered "unlimited talk, text, and Web or e-mail access for $50 a month with no penalty."

However, that doesn't necessarily mean that people are unhappy with their current service. Infogroup said that about 57 percent of respondents claimed they were "satisfied" with their monthly mobile phone bill, and did not plan to "switch because of cost."

But the company's expectation of growth in prepaid customers seems to follow trends already occurring in the mobile market.

Last month, AT&T announced that during its third quarter, it added 321,000 prepaid customers to its service. T-Mobile, which posted its third-quarter earnings today, saw prepaid net customer additions of 197,000 during the same period.

Infogroup/ORC conducted its study by surveying 715 wireless customers in the U.S. between October 7 and October 10.

Further reading: Prepaid smartphones to watch