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One in six Americans was a member of the Amazon Prime service at the end of the most recent holiday season, a 35 percent boost from a year before, according to a survey from a retail research firm released Monday.

The report, by Chicago-based Consumer Intelligence Research Partners, says that nearly half of Amazon.com's U.S. customers, or 54 million people, have signed up for Prime, a program that offers free two-day shipping for many purchases, as well as services such as streaming video and music. Members pay $99 a year. The 54 million figure, if true, could put Amazon Prime's U.S. enrollment in the territory of Seattle-area rival Costco Wholesale, which has 83 million cardholders among 45 million annual fee-paying households across its global operations.

Costco doesn't break down how many of those members are in the U.S., but about 70 percent of its warehouses are in the U.S. and Puerto Rico. Based on those figures, it could be assumed that about 58 million of its cardholders are in the U.S.

CIRP data is based on a survey of 500 U.S. participants who bought items from Amazon between October and December. Amazon is vague about how many people are part of the program. "There are tens of millions of Prime members," a spokeswoman said Monday.

Those subscribers spend nearly $1,100 per year at the online retailer, nearly twice as much as non-members do, according to CIRP.

The 35 percent increase more or less coincides with Amazon's claim earlier this month that orders on Cyber Monday (the online retail holiday that follows the Thanksgiving weekend) rose 40 percent year over year.

CIRP's figure is also not far from a report by RBC Capital Markets analysts last September saying that U.S. membership in the Prime program had risen to 40 percent of all U.S. customers.

CIRP says Amazon's growth spurt stemmed partly from the success of Amazon Prime Day on July 15, a summer, online version of Black Friday the tech giant enacted to celebrate its 20th anniversary.