Nearly 230 million children under the age of five around the world have never had their birth registered, which often means they will be barred from education, health care and social security, the UN children's agency said.
A report by UNICEF to mark its 67th birthday on Wednesday said globally that amounts to one in three children under five.
Last year, it said, only around 60 percent of all babies born were registered at birth, with the lowest levels of registration in south Asia and sub-Saharan Africa.
UNICEF Deputy Director Geeta Rao Gupta said birth registration not only acknowledges a child's identity and existence but is key "to guaranteeing that children are not forgotten, denied their rights or hidden from the progress of their nations."
"Birth registration and a birth certificate is vital for unlocking a child's full potential," Rao Gupta said. "If societies fail to count them, and don't even recognize that they are there, they are more vulnerable to neglect and abuse. Inevitably, their potential will be severely diminished."
UNICEF said failure to count children not only often excludes them from accessing education, health care and social security but affects the development of their communities and countries.
It cited many barriers to birth registration, from parents who are unaware of its importance to cultural barriers and fear of the consequences of reporting a birth including the misuse of personal information such as race, religion or birth out of wedlock.
It said children living in rural or remote areas, from poor families, and born to uneducated mothers are most likely not to be registered.
UNICEF called for programs to address the reasons families don't register children, including high fees and lack of awareness of laws and procedures.
The 10 countries with the lowest birth registration levels, according to UNICEF, are: Somalia (3%), Liberia (4%), Ethiopia (7%), Zambia (14%), Chad and Tanzania (16%), Yemen (17%), Guinea-Bissau (24%), Pakistan (27%) and Congo (28%).
AP