Secular groups have slammed Republican presidential candidate Ted Cruz, who recently said that anyone who wants to be President must not be an atheist.

Speaking at the National Religious Liberties Conference in Iowa last Friday, Cruz went on to say how important it was for the President of the US to pray everyday.

"Any President who doesn't begin every day on his knees isn't fit to be commander-in-chief of this country," said Cruz.

"We're a nation that has enjoyed God's blessing, providential blessing, from the very beginning," he added.

Cruz is a regular feature at conservative Christian gatherings since his election to the US Senate in 2012. He stopped short of explicitly saying non-believers, or rather those who don't pray every morning, should not hold any public office. Nevertheless Cruz has incurred the wrath of non-religious groups.

"By claiming that those who do not pray are not fit for office, Senator Cruz is perpetuating the prejudiced myth that humanists and atheist are not moral people," said Bishop McNeill from the Centre of Freethought Equality, a secular lobby group.

"There are a number of recent studies that show that despite how religious Americans remain, a majority of people would vote for a qualified atheist for president," continued McNeill.

"In his callous dismissal of humanists, atheists and other secular Americans, Senator Cruz also shows that he is out-of-touch with the growing number of Americans who are not affiliated with any religion."

Research has revealed that atheists are one of the least trusted groups in America with only socialists ranking below atheists. The Gallup study in June found that Americans were more likely to vote for any other group, including homosexuals or Muslims than vote for an atheist.

The research found that only 58 per cent of Americans would vote for an atheist, a 4 per cent rise from a similar study in 2012, showing that Cruz's remarks are not wholly out of line with the American public.

However atheists are also the most politically under-represented group in the US. Three per cent of Americans identify as atheist and four per cent as agnostic within a group of 23 per cent who are "unaffiliated" with any religion, according the recent Pew research. No members of Congress are openly atheist and only a handful of US politicians say they are "unaffiliated" to any religion.