For the first time since European settlement the US does not have a Protestant majority, according to a study, with the number of Americans claiming no religious affiliation on the rise.

The percentage of Protestant adults in the US has reached a low of 48%, the first time Pew Forum on Religion & Public Life has reported with certainty that the number has fallen below 50%.

The drop has long been anticipated and comes at a time when there are no Protestants on the supreme court and the Republicans have their first presidential ticket with no Protestant nominees.

Among the reasons for the change are the growth in nondenominational Christians who can no longer be categorised as Protestant, and a spike in the number of American adults who say they have no religion. The Pew study, released on Tuesday, found about 20% of Americans said they have no religious affiliation, an increase from 15% in the past five years.

Scholars have long debated whether people who say they no longer belong to a religious group should be considered secular. While the category as defined by Pew researchers includes atheists, it also encompasses majorities of people who say they believe in God, and a notable minority who pray daily or consider themselves "spiritual" but not "religious".

Still, Pew found overall that most of the unaffiliated were not actively seeking another religious home, indicating that their ties with organised religion were permanently broken.

Growth among those with no religion has been a major preoccupation of American faith leaders, who worry that the US, a highly religious country, would go the way of western Europe, where church attendance has plummeted.

Pope Benedict XVI has partly dedicated his pontificate to combating secularism in the west. This week in Rome he is convening a three-week synod of bishops from around the world aimed at bringing back Roman Catholics who have left the church.

The trend also has political implications. American voters who describe themselves as having no religion vote overwhelmingly for Democrats. Pew found Americans with no religion supported abortion rights and gay marriage at a much higher rate than the US public at large. These "nones" are an increasing segment of voters who are registered as Democrats or lean toward the party, growing from 17% to 24% over the past five years. The religiously unaffiliated are becoming as important a constituency to Democrats as evangelicals are to Republicans, Pew said.

The Pew analysis, conducted with PBS's Religion & Ethics Newsweekly, is based on several surveys, including a poll of nearly 3,000 adults conducted from 28 June-9 July.

The finding on the lack of a Protestant majority is based on responses from a larger group of more than 17,000 people and has a margin of error of plus or minus 0.9 percentage points, Pew researchers said. Pew said it had previously calculated a drop slightly below 50% among US Protestants, but those findings had fallen within the margin of error: the General Social Survey, which is conducted by the University of Chicago's National Opinion Research Centre, reported for 2010 that the percentage of US Protestants was around 46.7%.

Researchers have been struggling for decades to find a definitive reason for the steady rise in those with no religion. The spread of secularism in western Europe was often viewed as a byproduct of growing wealth in the region. Yet among industrialised nations, the US stood out for its deep religiosity in the face of increasing wealth.

Religion scholars now say the decreased religiosity in the US could reflect a change in how Americans describe their religious lives. In 2007, 60% of people who said they seldom or never attended religious services still identified themselves as part of a particular religious tradition. In 2012, that statistic fell to 50%, according to the Pew report.

"Part of what's going on here is that the stigma associated with not being part of any religious community has declined," said John Green, a specialist in religion and politics at the University of Akron, who advised Pew on the survey. "In some parts of the country, there is still a stigma. But overall, it's not the way it used to be."

The study has found the growth in unaffiliated Americans spans a broad range of groups: men and women, university graduates and those without a university degree, people earning less than $30,000 annually and those earning $75,000 or more. However, along ethnic lines, the largest jump in "nones" has been among white people, with one-fifth describing themselves as having no religion.

More growth in "nones" is expected. One-third of adults under the age of 30 have no religious affiliation, compared with 9% of people aged 65 and older. Pew researchers wrote that "young adults today are much more likely to be unaffiliated than previous generations were at a similar stage in their lives", and aren't expected to become more religiously active as they age.