South African Archbishop Desmond Tutu has helped launch a global campaign calling for governments to ensure all children are registered at birth.
Governments are urged to ensure all children are registered
He said it was a matter of life and death - an unregistered child did not officially exist and was vulnerable to traffickers and during disasters.
In South Asia alone, there are no records for six out of every 10 babies, campaign organisers Plan say.
The agency fears around half a billion children worldwide may be unregistered.
Archbishop Tutu said a birth document was important because it "proves who you are". Without it children are often barred from education, health care and citizenship.
"It is, in a very real sense, a matter of life and death," the Nobel Peace Prize laureate told a news conference at the UN headquarters in New York.
"The unregistered child is a nonentity. The unregistered child does not exist. How can we live with the knowledge that we could have made a difference?"
The UN Convention on the Rights of the Child imposes an obligation on countries to register every child immediately after birth.
But Plan, the international agency organising the registration campaign, said that was not happening in many parts of the world.
An unregistered child does not officially exist: Desmond Tutu
In a report released to coincide with the campaign, Plan said no records existed for 60% of babies born annually in South Asia, and that 55% of births in sub-Saharan Africa go unrecorded.
"Governments worldwide are failing the world's children, as millions of youngsters without a birth certificate find it very difficult to prove their age or nationality," said Thomas Miller, Plan's chief executive.
"And parents whose children go missing during disasters like the tsunami or because they are abducted by traffickers may even be unable to get help with tracing their sons or daughters because they cannot prove the age of their children - or in many cases that their children even exist."
He said a recent campaign in Cambodia - in which they registered 2.4 million people in less than four months - showed it could be done without incurring high costs.