WASHINGTON — Protestants no longer make up the majority of the US population, while the proportion of Americans who claim no religious affiliation is at an all-time high, the Pew Research Center said Tuesday.

In a report, the Washington think tank's Forum on Religion & Public Life said 48 percent of American adults currently identify themselves as Protestant, down from 53 percent five years ago.

"This marks the first time in Pew Research Center surveys that the Protestant share of the population has dipped significantly below 50 percent," it said.

"The decline is concentrated among white Protestants, including those who consider themselves born-again or evangelical Protestants as well as those who do not."

Over the same five years, the proportion of those with no religious affiliation -- the so-called "nones" -- has grown from just over 15 percent in 2007 to a record high of just under 20 percent, Pew said.

That includes more than 13 million self-described atheists and agnostics, and nearly 33 million who say they have no particular religious affiliation despite, in most cases, believing in God.

"The growth in the number of religiously unaffiliated Americans ... is largely driven by generational replacement, the gradual supplanting of older generations by newer ones," Pew said.

Thirty-two percent of adults under 30 have no religious affiliation, compared with nine percent of over-65s, and they are "much more likely" to be unaffiliated than previous generations were at a similar stage in their lives.

Pew said its report was based on an analysis of dozens of surveys it has conducted in recent years among tens of thousands of respondents.

[A Bible and a crucifix via Shutterstock]