The number of US Hispanics identifying themselves as Catholic has declined in the last five years.

According to a Gallup poll, 11 per cent of US Hispanics selected their religious preference as "None/Don't Know Refused" in 2008, compared with 15 per cent in 2012.

While the number identifying themselves as Protestant remained largely the same (27 per cent in 2008 versus 28 per cent in 2012), those identifying themselves as Catholic fell in the last five years from 58 per cent to 54 per cent.

Older Hispanics were more likely to be Catholic, while less than half of 18 to 29-year-old Hispanics (47 per cent) said they were Catholic.

Eighteen to 29-year-olds also accounted for the largest proportion of Hispanics without any religious identity (20 per cent).

"The Protestant percentage is almost identical across all age groups. Younger Hispanics, as is the case in general in the US population, are less likely to have any religious identity at all," said a Gallup report.

"This tendency of the youngest Hispanic Americans to be 'nones' - without an explicit religious identity - thus appears to come at the cost of Catholic identity and not Protestant identity."

Protestant Hispanics were significantly more likely to be religious (60 per cent) compared to Catholics (43 per cent).

The gap in religiosity is evident across Hispanics of all ages. While a third of 18 to 29-year-old Hispanic Catholics describe themselves as very religious, this rises to over half (52 per cent) of Hispanic Protestants in the same age bracket.

The Gallup report added: "Overall, the finding that younger Hispanics are proportionately more Protestant and that all Hispanics are becoming proportionately more Protestant over time suggest that the percentage of Hispanics who are Catholic may continue to slip in the years to come."

The findings are based on a survey of more than 28,000 US Hispanics.