Almost half of the population of England and Wales identifies itself as having no religion, outweighing the number of people who say they are Christian, research found.
In 2014, 48.5% of those asked referred to themselves as having no religion, compared to the 25% that fell into the "none" category in the 2011 census.
Those who defined themselves as Christian - Anglicans, Catholics and other denominations - made up 43.8% of the population, the Guardian reports.
Stephen Bullivant, senior lecturer in theology and ethics at St Mary's Catholic University in Twickenham, told the newspaper there was a "clear sense of the growth of 'no religion' as a proportion of the population".
He said: "The main driver is people who were brought up with some religion now saying they have no religion.
"What we're seeing is an acceleration in the numbers of people not only not practising their faith on a regular basis, but not even ticking the box.
"The reason for that is the big question in the sociology of religion."
He analysed data collected through British Social Attitudes surveys over three decades, but the report, Contemporary Catholicism in England and Wales, did not look at data from Scotland or Northern Ireland.
According to Dr Bullivant's report, which will be launched at the House of Commons on Tuesday, the Anglican and Catholic churches are struggling to retain people brought up as Christians.
The proportion of the population describing themselves as Anglican plunged from 44.5% in 1983 to 19% in 2014. Catholics made up 8.3%, other Christians 15.7% and non-Christian religions 7.7%.