A sign for the Apple Pay payment system is seen at the entrance to a McDonald's restaurant in London, July 14, 2015. Chris Ratcliffe/Bloomberg via Getty Images

SAN FRANCISCO -- Americans are no longer signing up for Apple Pay as much as they used to, a marketing firm that has analyzed the Apple payment option announced Monday. At the end of September, only 14 percent of American households with credit cards had signed up for the service, up only three percentage points since February and indicating slowing growth for the payment option.

The figures were reported by Phoenix Marketing International, a marketing consulting firm, at a payments conference in Las Vegas. "A very rapid initial threshold was achieved by Apple Pay, and it is still growing but the growth rate has slowed down," said Greg Weed, Phoenix Marketing's director of card performance research, Reuters reported.

Apple Pay is in a crowded market with many players, including Google's Android Pay, Samsung Pay and, soon, a new challenger from JP Morgan Chase.

The marketing firm's research also found that among Apple Pay users, 86 percent signed up with credit cards while 49 percent use debit cards and 22 percent use other types of prepaid cards. Phoenix Marketing has been tracking Apple Pay since the service's launch a year ago by following a group of 15,000 consumers, Reuters said.

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Fortunately for Apple, the tech company is still doing a good job getting more retailers to sign up for Apple Pay, which lets iPhone and Apple Watch owners pay using their mobile devices. Earlier this month, Apple added Best Buy to its list of retailers now accepting Apple Pay. Additionally, the company announced that Starbucks, Chili's and KFC will also soon begin accepting the payment option.

Apple is set to deliver its fourth-quarter results Tuesday at 5 p.m. EDT.