With 44 million households having used the Internet to pay a bill in the past 30 days — up from 32 million five years ago and projected to reach 55 million by 2016 — it’s a shift that has major ramifications for competition.

There’s even evidence that fewer consumers are switching banks, with 7 percent of them estimated to be moving their primary account to a different institution in 2011, down from 12 percent last year, according to surveys by Javelin Strategy and Research.

Emmett Higdon, a consultant who managed Citibank’s online bill payment product from 2004 to 2007, said that “for the consumer, it’s a double-edged sword.” While customers value the convenience, inside the industry “it was known that it would be a powerful retention tool. That’s why online bill paying went free in the first place. Inertia is powerful in the banking industry.”

Bank of America today has 29 million account holders banking online and 15 million using the service to pay bills, but company officials say there is no connection between the stickiness of Internet bill paying and the decision to impose the $5 monthly debit card fee.

“People like online bill pay, it’s convenient and safe,” said Anne Pace, a spokeswoman for the company. “The lower attrition rate that came along with it was simply a result of offering a valuable service.”

The fee, she said, “allows us to continue offering the benefits that customers have come to expect from our debit card,” like fraud protection, overdraft prevention and a wide-reaching A.T.M. network.

Asked if the bank calculated how many online-bill-pay customers a new fee could drive away, Ms. Pace said, “We did extensive research on how they would react to a new fee and whether it was fair.”