There may be more than twice as many atheists in the US than previous studies have found, according to a new study that takes into account people's tendency to lie about whether they believe in God.

The report by two University of Kentucky scholars suggests that because people may be embarrassed to admit they don't believe, the number of Americans who say they are non-believers may be artificially low, according to Religion News Service (RNS).

Polls from Gallup, Pew and Barna have reported that number as being between three and 10 per cent. But the real number of American atheists may be as high as 26 per cent, according to the psychologists Will Gervais and Maxine Najle.

'There's a lot of atheists in the closet,' Gervais told Vox. 'And...if they knew there are lots of people just like them out there, that could potentially promote more tolerance.'

Gervais and Najle's research, which will be published in the next issue of the journal Social Psychological and Personality Science, shows that people are less likely to be truthful when answering a direct 'yes or no' question about their faith, such as, 'Do you believe in God?'

Two groups of people were polled, with each asked how many of several statements were true for them, such as 'I am a vegetarian,' 'I own a dog,' and 'I believe in God'.

One group – 2,000 people – received questionnaires with no query about God and another group – 1,000 people – questionnaires that asked about belief in God, RNS explained.

By comparing the two groups, Gervais and Najle conclude that the number of American atheists is usually under-reported by most polls.

'According to our samples, about one in three atheists in our country don't feel comfortable disclosing their lack of belief,' Najle told Vox.