ATHEISTS and people not affiliated to any religion will find themselves in a shrinking minority over the next four decades, as the faithful soar in number, projections in a report by Pew suggest.

Christians will remain the largest religious group in 2050, growing in line with the global population, and thus remaining at 31 per cent. Islam will grow by 73 per cent, faster than any other great religion, and will make up 29.7 per cent of the global population (up from 23.2 per cent in 2010).

Although the atheists and unaffiliated will increase in absolute number - from 1.1 billion to 1.2 billion - they will represent 13 per cent of the world's population, down from 16 per cent in 2010. With the exception of Buddhists, all the main religious groups are set to grow in absolute numbers.

Pew's report, The Future of World Religions, published this month, suggests that the picture is largely determined by differences in fertility rates and age profiles: "Religions with many adherents in developing countries, where birth rates are high and infant morality rates generally have been falling, are likely to grow quickly."

Muslims have the world's highest fertility rate - 3.1 children per woman compared with a global average of 2.5 - and 34 per cent are under the age of 15. Christians have, on average, 2.7 children, and 27 per cent are under 15. The non-religious are largely concentrated in places that have low fertility and ageing populations, with 1.7 children on average, and 19 per cent under 15.

There are "vivid geographical differences", the report notes. Much of the growth in Islam and Christianity is expected to take place in sub-Saharan Africa, where 40 per cent of Christians in the world will live by 2050. The proportion of the population which is unaffiliated to any religion is set to grow in much of Europe and the United States. In the latter, it is forecast to grow from 16 per cent to 26 per cent.

The report estimates that, by 2050, ten per cent of people in Europe will be Muslims. The absolute number of Christians is expected to decline, from 553 million to 454 million. In the UK, the percentage of Christians is projected to fall from 64.3 per cent to 45.4.

Of all religious groups, Christians are expected to experience the largest net loss from people who switch religions; most of these will join the unaffiliated. Just 30 per cent of people polled in the UK describe themselves as "religious", another poll suggests. More than half (53 per cent) said that they were "not religious", and 13 per cent called themselves "convinced" atheists. The remainder said "don't know".

WIN Gallup International interviewed 63,898 people across the world at the end of last year. Almost two-thirds (63 per cent) described themselves as "religious", 22 per cent said that they were not, and 11 per cent defined themselves as convinced atheists.

Africa and the Middle East were the most religious regions: respectively, 86 and 87 per cent of people there described themselves as religious. This compares with 56 per cent in America and 70 per cent in Russia. In China, 61 per cent of people claimed to be convinced atheists - almost twice as many as in any other country.

Those under 34 tended to be more religious than their elders (66 per cent compared with 60 per cent). Those without an education were the most religious (80 per cent). Income "appears to exert a greater influence" than education, the report suggests. Among those with medium-high and high incomes, less than 50 per cent said they were religious, compared with 70 per cent of those with low, medium-low, and medium incomes. The number of convinced atheists is as high as 25 per cent among people with high incomes, but only six per cent among those with low incomes.