(Reuters) - One out of five U.S. adults do not identify with a specific religion, and the number of Protestants has for the first time dipped significantly below 50 percent, according to a poll released on Tuesday by the Pew Research Center.

Pew researchers said the growing ranks of those with no religious affiliation suggests that Americans slowly may be becoming less religious.

The number of adults who consider themselves atheist, agnostic or "nothing in particular" has increased from about 15 percent to nearly 20 percent in the last five years, the highest percentage ever for that group in Pew polling.

Fifty-eight percent of Americans still say religion is very important in their lives, far exceeding the value given to religion by people in Britain, France, Germany or Spain, Pew said.

The survey by the Pew Research Center's Forum on Religion & Public Life also found that many of the country's 46 million adults who don't claim a religion remain religious and spiritual in some way even though most are not seeking a denomination.

Seventy-three percent of U.S. adults are Christian, but the number of Protestants dropped to 48 percent in 2012 from 50 percent in 2011 and 53 percent in 2007, the poll showed. Pew said it was the first time its surveys had found Protestants to no longer make up at least half of the overall population.

Recently the religiously unaffiliated have increasingly voted Democratic, Pew said, with exit polls conducted by a consortium of news organizations showing a rise from 61 percent in the 2000 presidential election to 75 percent in 2008.

Twenty-four percent of the religiously unaffiliated were Democratic or Democratic-leaning registered voters in 2012, compared with only 11 percent of the unaffiliated who said they were Republican or Republican-leaning voters.

Pew said the findings were based on several surveys, including most recently a national telephone poll of 2,973 adults that was conducted between June 28 and July 9, 2012 and an additional 511 interviews with religiously unaffiliated adults between June 28 and July 10.

(Reporting by Colleen Jenkins; editing by Prudence Crowther)