Over 19 lakh people will have to prove citizenship after they were left out of Citizens' List.

The Assam Public Works (APW), the original petitioner in the Supreme Court which led to the updation of the National Register of Citizens (NRC) six years ago, said the final NRC turned out to be a "flawed document" because its prayer for reverification of the draft list was rejected by the top court.

The NGO also wondered whether the software used in the updation exercise was capable to handle so much data and if it was examined by any third party Information Technology expert, APW president Aabhijeet Sharma said.

"The final NRC has made it obvious that the problem of illegal immigration will never be resolved in Assam. Had it been completed flawlessly, it would have been a golden chapter in Assam's history," Mr Sharma said after the final NRC was released, excluding names of over 19 lakh applicants.

The APW, as the primary petitioner, had submitted five memoranda in the Supreme Court requesting re-verification of the draft NRC but they were rejected, he said.

"The 27 per cent re-verification done by him (NRC State Coordinator Prateek Hajela) is a mystery. No one knows whether or not it was 100 per cent flawless," Mr Sharma said.

He also expressed "strong doubt" on the software used as names of many doubtful voters had entered the draft NRC. "Was it due to the flaws of the software that members of 39 families of 'D' (doubtful) voters of Morigaon district are included in the draft NRC as mentioned by the then district dommissioner?" the APW president asked.

In 2009, the APW filed a petition in the Supreme Court praying that names of 41 lakh foreigners be deleted from the electoral rolls of Assam and the NRC updated.

In response to the petition, the Centre told the top court that the 1951 NRC was being updated. The Supreme Court in 2013 took up the APW petition and directed both the Central and state governments to begin the process for updating the NRC and the actual work began two years later.