Unicef: 230m young children 'do not officially exist'
The UN children's charity, Unicef, says one in three children under five have not had their births registered.
In its report, Unicef says nearly 230 million children worldwide do not officially exist, making them more vulnerable to neglect or abuse.
It says a birth certificate guarantees that children are not denied rights or basic services such as education.
The countries with the lowest registration levels are in South Asia and sub-Saharan Africa.
"Birth registration is more than just a right. It's how societies first recognize and acknowledge a child's identity and existence," said Geeta Rao Gupta, Unicef Deputy Executive Director.
"Birth registration is also key to guaranteeing that children are not forgotten, denied their rights or hidden from the progress of their nations."
Unicef analysed data from 161 countries in compiling its report, Every Child's Birth Right: Inequalities and trends in birth registration.
Mobile phones helping
It said that in 2012, only about 60% of all babies born worldwide were registered at birth.
The rates varied significantly even within continents, with fewer than one in ten births registered in Somalia, Liberia and Ethiopia, while in South Africa 95% are recorded - approaching levels in G8 countries.
Unicef said it was using innovative approaches to help governments and communities strengthen their civil and birth registration systems.
For example, in Uganda, the government - supported by the UN and the private sector - has introduced mobile phone technology "to complete birth procedures in minutes, a process that normally takes months", says the report.
Unicef says that in East and Southern Africa, only about half of those children registered actually have a birth certificate.
Prohibitive fees, unawareness of the relevant laws or processes, cultural barriers, and the fear of further discrimination, are all listed as reasons why families do not register children.
The UN agency says children unregistered at birth or without identification documents are often excluded from access to education, health care and social security.
And if children are separated from their families during natural disasters, conflicts or as a result of exploitation, reuniting them is made more difficult by the lack of official documentation, it says.
"Birth registration - and a birth certificate - is vital for unlocking a child's full potential," said Mrs Rao Gupta.
"All children are born with enormous potential. But if societies fail to count them, and don't even recognise that they are there, they are more vulnerable to neglect and abuse."