It was billed as a way to stamp out corruption and transform social welfare in the world’s most populous democracy by showering the benefits of modern technology on hundreds of millions of Asia’s most impoverished people.

But India’s plan to give its 1.4 billion people biometric digital ID cards has proved far more complex than originally anticipated.

Now the so-called Aadhaar scheme (Hindi for ‘foundation’ or ‘base’) has become the focus of a ferocious legal battle, pitting privacy campaigners against some of India’s biggest and wealthiest companies.

With 99pc of Indian adults enrolled in the scheme, Aadhaar is easily the world's largest biometric ID database.

Launched in 2009 by Nandan Nilekani, one of the founders of Indian technology outsourcing giant Infosys, it was originally viewed as a way of registering Indians - millions of whom lack birth certificates or other ID - so they could access their rightful welfare payments and prevent the mass pilfering of state subsidy schemes.

In exchange for providing fingerprints and retina scans, people who sign up for Aadhaar get a 12-digit ID number.

The cards cover welfare and tax payments as well as the ability to access social services.

Despite humble beginnings, over time the Aadhaar scheme - which when India holds its next general election in 2019 will mark its tenth birthday - has evolved into an all-encompassing ID scheme for all Indian citizens, used for everything from filing taxes to registering marriages.