India has released census figures showing that the Muslim community is growing much faster than other groups.
Rapid growth but poor literacy rates characterise India's Muslims
Hindus continue to make up about 80% of India's billion-plus population but their growth rate has declined.
This is the first time a religious breakdown of India's population figures has assessed "the progress made by different groups".
But critics have questioned the timing of the survey, saying it could be exploited by Hindu hardliners.
According to figures released by India's census commissioner, India's Muslim community grew by 36% between 1991 and 2001 and now stands at 138 million, or 13.4% of the total population.
Hindus account for 80.5% of all Indians, a growth of 20.3% in the same period, down from 25% in 1981-1991.
Christians make up the third largest group (24 million) followed by Sikhs (19 million).
The Muslim community fares poorly in literacy compared to other groups - which is seen as one reason for their increasing numbers.
"There has not been a significant increase in Muslim population in the southern state of Kerala because of high literacy rates," the chairman of India's Minorities Commission, Tarlochan Singh, told the BBC.
Hindus: 827.5m - 80.5%
Muslims: 138.2m - 13.4%
Christians: 24m - 2.3%
Sikhs: 19.2m - 1.9%
"But unfortunately this is not the case with their population in other parts of the country."
There were other alarming numbers for the Sikhs and the small Parsi community.
The ratio of females to males was lowest among the Sikh community - 893 women for every 1,000 males.
And despite controlling more than 15% of the market value in India's main financial market, the Bombay Stock Exchange, the Parsi community makes up less than 1% of Indians and is on the verge of extinction because of its low birth rates.
The Parsi population has decreased from 76,382 in 1991 to 69,601 in 2001.
The timing of the census figures is being criticised by some who say it could be used politically against Muslims.
"I am absolutely sure that right-wing Hindu groups will misuse this data politically," social activist Teesta Setalvad told the BBC.
Fears that Hindu hardliners will exploit the findings
In its first reaction, the main opposition Hindu nationalist BJP said the increased numbers of Muslims was a matter of "grave concern for all those who think of India's unity and integrity in the long term".
"We are strongly in favour of an even and uniform adoption of population control measures by people belonging to different communities," party president Venkaiah Naidu said.
"Any imbalance in this regard is not a healthy trend."