The Indian government is in the midst of enrolling all of its citizens in the world’s largest domestic biometric identification system. Aadhar, which means foundation, started as a voluntary programme to help Indians get food subsidies and welfare benefits.

But the government has now made it mandatory and wants the ID to be linked to things like tax returns, phone numbers and bank accounts. More than a billion cards have been issued, but the scheme is deeply controversial. Justin Rowlatt explains.

Produced and edited by Prem Boominathan.