The US is becoming increasingly secular, with the number of Americans without any religion increasing year after year.

At the same time, fewer Americans are identifying as Christian, according to polling by Pew Research Center. 

Around three in 10 US adults (29%) are religious "nones" - those who identify as atheist, agnostic or "nothing in particular" - 6 percentage points higher than five years ago and 10 points higher than a decade ago. 

Self-identified Christians account for just under two thirds (63%) of the adult population - down from over three quarters (78%) in 2007. 

The poll of over 3,900 US adults found the decline most concentrated among Protestants. Four in 10 Americans identify as Protestant, down 4 percentage points in five years and 10 points in the last decade. 

By comparison, just over a fifth (21%) describe themselves as Catholic, the same proportion as in 2014. 

Of those who identified as Protestant, a majority (60%) said they thought of themselves as "born-again or evangelical Christian". 

Overall, the proportion of Americans who identify as evangelical Protestant has fallen by 6 percentage points since 2007 to just under a quarter (24%) of US adults. 

Protestants who do not identify as evangelical have experienced a similar decline from 22% to 16% in the same time frame. 

The number of Americans who say they pray daily has also steadily fallen in the last 14 years from over half the population (58%) to 45% today. 

Mirroring this change, only four in 10 Americans say religion is "very important" in their lives, compared with 56% who said the same in 2007. 

"The secularizing shifts evident in American society so far in the 21st century show no signs of slowing," Pew said.