ATLANTA – When millions switch on their new smartphones for the first time on Christmas Day, many of them will find themselves in unfamiliar but fast-growing ­territory.

No longer will they be tied to a two-year contract.

A growing number of consumers are moving into the world of "prepaid" — that is, no-contract plans — in efforts to save money and to feel less tied down to one company or wireless device. Once known only as the wireless plan for consumers who had bad credit or weren't able to pay their phone bills on time, no-contract plans are becoming more popular with all consumers.

Instead of offering only one or two prepaid plans that sell bare-bones service at a higher rate, wireless companies now have a variety of prepaid plans that include texting and data options. Those options are expanding to let consumers roll over talk and data minutes to the next month, for example.

"It was the 'low-rent' kind of service, but the reality is that's not true anymore," said Jeff Blyskal, a senior editor with Consumer Reports. "It's become easier and cheaper than contract service."

No-contract plans now make up nearly 25 percent of all mobile phone subscribers, according to analyst reports. The number of consumers with wireless service without a contract hit 100 million in 2012, up 12 percent from the year before, according to reports.

"Now that everybody has moved to a smartphone, people are saying, 'Let's find a cheaper way,' " said Roger Entner, a telecommunications analyst with Boston-based Recon Analytics.

For years, such companies as TracFone Wireless, Cricket Communications, and Boost Mobile, a subsidiary of Sprint, have been some of the popular providers of prepaid cellphones. Republic Wireless is now making headlines with its Moto X device that runs over Wi-Fi when possible.

The large wireless carriers are ramping up their presence in the no-contract market. Atlanta-based AT & T Mobility started up Aio Wireless in May. AT & T, the nation's No. 2 mobile phone carrier behind Verizon Wireless, also plans to buy Leap Wireless, which owns the Cricket brand, in a deal expected to close during the first three months of 2014.

There are several reasons for the expansion of prepaid options, said Nataraj Rao, Aio's head of business operations and strategy. A saturated smartphone market has caused consumers to want more flexibility in their calling plans. Consumers who qualify for only prepaid services were clamoring for more devices and features so they could use the Internet, apps and other technologies.

"It doesn't mean you have to settle for bare bones," Rao said.

Aio's plans are at $40, $55 or $70, depending upon how much high-speed data access a consumer wants.

The switch to no-contract plans also is a signal that the wireless market is shifting its business model away from one that uses subsidized smartphones to hook customers and tie them to a two-year contract. Wireless carriers are proactively talking up their no-contract plans to get away from the notion that they force ­consumers to sign on for something they don't want.

Consumers typically would pay $100, or sometimes nothing, for a new smartphone that actually costs the wireless provider $500 or more. They spend the next two years paying off that phone as part of their monthly wireless bill.