The Indian government has approved the inclusion of information on caste in the ongoing population census.
The controversial decision was taken by a group of ministers, headed by Finance Minister Pranab Mukherjee.
Caste-based parties say the information will help the government target affirmative action benefits better.
But critics say caste is the most regressive feature of Indian society; that it is repressive, reinforces hierarchy and breeds inequity.
India has been conducting the national census since 1872 and this is its 15th.
A caste census was carried out last in 1931 by the former British rulers.
The census, launched in April, will include photographs and fingerprints of everyone aged over 15 to create a biometric national database.
The government will then use the information to issue identity cards.
Officials will spend a year classifying India's population of around 1.2 billion people according to gender, religion, occupation and education.
The exercise, conducted every 10 years, faces big challenges, not least India's vast area and diversity of cultures.
Census officials must also contend with high levels of illiteracy and millions of homeless people - as well as insurgencies by Maoists and other rebels which have left large parts of the country unsafe.
Over the next year, some 2.5 million census officials will visit households in more than 7,000 towns and 600,000 villages.
The mammoth registration exercise will stretch over 11 months, consume more than 11 million tonnes of paper, and cost 60bn rupees ($1.3bn; £880m).
The national census is the only source of primary and credible data in India and is used not just to formulate government policies but also by private companies to identify markets for their products.
The full census results will be released in mid-2011.