According to census data analysis published yesterday, the number of British-born Christians is falling significantly, whilst the number of young Muslims is on the rise.

The figures suggest Christianity is in long-term decline in the UK; there are 5.3 million fewer British-born people describing themselves as Christians, representing a decline of 15% in just a decade, despite a growth in the overall population, calling into question the establishment of the Church of England.

Notably, the proportion of young people who describe themselves as even nominal Christians has dropped below half for the first time. Younger people also drove a shift away from religion altogether, with 6.4 million more people describing themselves as having no religion than 10 years earlier.

Whilst initial results from the 2011 census, published last year, showed that the total number of people in England and Wales who described themselves as Christian fell by 4.1 million, equaling a decline of 10%, this figure masked the fact that for British-born Christians the decline was even larger. This is because the figures had been bolstered by 1.2 million foreign born Christians, including Polish Christians and evangelical Christians from places such as Nigeria.

Keith Porteous Wood, executive director of the National Secular Society, commented: "Realistically, this trend is irreversible, and the number has dropped below critical mass for which there is no longer any justification for an established Church".

Whilst there has been a drop in those subscribing to the Christian faith, the number of Muslims in England and Wales has surged by 75% – boosted by almost 600,000 more foreign born followers of the Islamic faith. While almost half of British Muslims are under the age of 25, almost a quarter of Christians are over 65. The average age of a British Muslim is just 25, not far off half that of a British Christian.

David Coleman, Professor of Demography at the University of Oxford, told The Times that the findings showed how Christianity was declining with each generation. "Each large age group, as time progresses, receives less inculcation into Christianity than its predecessor ten years earlier," he said.

Professor Coleman contrasted the decline of the Christian faith through the generations with what happens among Muslims. "We have a Muslim faith where most studies suggest adherence to Islam is not only transmitted through the generations but appears to get stronger," he said. "Indeed, there seems to be some evidence that the second generation Muslims in Britain are more Muslim than their parents."

A spokesman for the Church of England acknowledged: "One of the reasons may well be fewer people identifying as 'cultural Christians', that is those who have no active involvement with churches and who may previously have identified as Christian for cultural or historical reasons.

Terry Sanderson, President of the National Secular Society, said that the Church was well aware of its parlous position which is why it is so anxious to reinforce its position in education; "The early indoctrination of children is very important for the continuation of any religion," he said. "Christians know this and Muslims know it. The Church of England has far more primary schools under its control than secondary ones. It knows it needs to start the process earlier in order to keep people in the fold.

"Muslim clerics also know that early indoctrination is essential, which explains the widespread use of madrassas – which, if we are honest, are little more than brainwashing institutions. There is little wonder that young Muslims are more religious than their parents with such heavy-handed, inescapable religious propaganda being forced on them."

Mr Sanderson said that the institutionalisation of secularism should now be an urgent priority for this government. "Within a few generations we are going to have a battle for supremacy between religions, and the non-religious are being further marginalised despite being a growing group. For the safety of us all we need to ensure that no religion can take control of the state again."

Key points

• In 2011, Christianity had the oldest age profile of the main religious groups.

• The number of Christians has fallen and this was largely for people aged under 60.

• The number of people with no religion has increased across all age groups, particularly for those aged 20 to 24 and the 40 to 44.

• In England and Wales, over nine in ten Christians (93%) were White and nine in ten (89%) were born in the UK, though the numbers have fallen since 2001.

• Nearly four in ten Muslims (38%) reported their ethnicity as Pakistani, a 371,000 increase (from 658,000 to over a million) since 2001. Nearly half of all Muslims were born in the UK.

• The majority of people with no religion were White (93%) and born in the UK (93%) and these groups have increased since 2001.

• People with no religion had the highest proportion of people who were economically active, Christians and Muslims the lowest. Jewish people had the highest level of employment and Muslim people the highest level of unemployment.

• The main reason for Christians being economically inactive was retirement, for Muslims economic inactivity was mainly because they were students, or because they were looking after the home or family.