India's Hindus have dropped below 80 per cent of the population for the first time since independence and media had speculated the previous government deliberately delayed the release of the data because it showed a rise in the Muslim population.

Members of Prime Minister Narendra Modi's ruling Hindu nationalist party, which swept to power last year, have expressed growing concern about the rising numbers of Muslims.

The census data shows that Hindus declined to 79.8 per cent of the country's 1.2 billion people in 2011, from 80.5 per cent a decade earlier.

The share of Muslims rose to 14.2 per cent from 13.4 per cent in 2001 - the only major religious group to record a rise. Christians stayed at 2.3 per cent and Sikhs fell to 1.7 per cent from 1.9 per cent.

Sakshi Maharaj, a Hindu priest-turned-politician, caused an uproar earlier this year when he said Hindu women should give birth to four children to ensure that their religion survives.

In the first census, conducted after Britain carved India and Pakistan out of colonial India in 1947, Hindus accounted for 84.1 per cent of the Indian population.

Although population growth is slowing in all religious groups, India is still set to overtake China to become the world's most populous country by 2022, according to a United Nations forecast.

India's population grew by almost a fifth during the period between the last two censuses, straining supplies of land, food and water and bloating its underemployed, poorly skilled workforce. - Reuters