The number of religious conservatives in the US is declining with every generation, a new report has concluded.

The study by the Public Religion Research Institute found that one in five Americans (19%) are religious progressives, while 28% are religious conservatives. Over a third (38%) were religious moderates and 15% were non-religious.

Although religious conservatives still outnumber religious progressives, the institute noted the latter were significantly younger and more diverse.

While the mean age of religious conservatives was 53, religious progressives were 44.

Among Millennials (those aged 18 to 33), just under a quarter (23%) were religious progressives, compared to 17% who were religious conservatives. Non-religious were similar in number to religious progressives (22%).

By contrast, among those aged 66 to 88, just 12% were religious progressives and only 10% were non-religious. Nearly half (47%) were religious conservatives.

"Our new research shows a complex religious landscape, with religious conservatives holding an advantage over religious progressives in terms of size and homogeneity," said Dr Robert P Jones, chief executive of the Public Religion Research Institute. "However, the percentage of religious conservatives shrinks in each successive generation, with religious progressives outnumbering religious conservatives in the Millennial generation."

White evangelical Protestants make up the highest proportion of religious conservatives (43%), followed by Catholics (17%), and white mainline Protestants (15%). Black Protestants account for around 9% of religious progressives and 8% of religious conservatives.

Religious progressives are more diverse in make up, with the largest group being Catholics (29%). This is followed by white mainline Protestants (19%), those who are unaffiliated with a religious tradition but still regard it as important in their lives (18%), and non-Christian religious Americans such as Jews, Buddhists, Hindus and Muslims (13%).

In stark contrast to their high representation among religious conservatives, white evangelical Protestants account for just 4% of religious progressives.

"Religious conservatives are a known quantity and they play an important role in our politics," said EJ Dionne, Brookings senior fellow.

"But this survey also shows that religious progressives are a more significant group than is usually assumed, and that there is a strong social justice constituency among religious Americans that cuts across labels."

Progressives and conservatives also differed in terms of what they believed it meant to be a religious person, with the majority of progressives (79%) saying it was mostly about doing the right thing, and the majority of conservatives saying it was primarily about having the right beliefs (54%).

The study's findings are based on surveys with of over 2,000 Americans.

However, its conclusions were challenged by the Institute on Religion and Democracy, which said that the rise of religious liberals in the US assumed Millennials would not change their beliefs as they age.

IRD President Mark Tooley said: "Many Millennials will become more religious and conservative as they age, especially if they marry and have children. Religious conservatives also have more children than religious liberals or secularists.

"The next generation of religious conservatives may speak differently from yesterday's Religious Right. But they will still be conservative and likely much more numerous than liberal counterparts.

"The Left often expects abstract tides of history to ensure its final victory. But religious liberals, who almost always dilute the doctrines of their faith to replace or supplement them with secular fads, usually ensure their own demise."

He pointed to the growth among theologically conservative congregations and seminaries, compared to the steady membership loss being seen within the liberal-leaning denominations.

"Liberal religion is usually a reaction against trans-generational orthodoxy, not a sustaining alternative to it. In contrast, religious orthodoxy is not a guarantee of vibrancy and growth but it almost always is a prerequisite," he said.