Guriya Khatun, 14, was once an “invisible” child. When she was nine and her father became ill, she was forced to toil in the fields near her village in Gaya, in the impoverished Indian state of Bihar. Even then, she understood that education was her only hope.

Against all the odds, and despite a four-mile walk every day, she now attends school thanks to a special bridging course sponsored by the United Nations children’s fund Unicef.

Two years ago, she was illiterate, but now she can read as well as other 14-year-olds.

At first her mother would not agree to her schooling. It was not the “tradition” in their small Muslim community.

But Guriya, mature beyond her years, stood her ground.

“My message is that you must never give up,” she said. “It is never too late.”

Unicef says there are millions of children suffering from severe exploitation and discrimination who are “invisible” to their communities, their governments and to the world.

In its annual report, State of the World’s Children 2006, the charity says governments must act now to bring these “invisible” children back from the brink.

It is an important human rights issue, but excluding children from education also sets back a country’s development.

The UN’s millenium development goals, which include Education For All, will not be achieved by 2015 unless the most vulnerable are identified and brought into the system, the charity says.

These “invisible” children fall through the cracks and are often deprived of education because they are not registered at birth, are orphaned, or forced into labour or early marriage. Others are fighting in conflicts or trafficked as slaves, or for sex.

The global problem has now been quantified for the first time. More than 50 million births - more than half of births in developing countries - remain unregistered every year, particularly in poor rural areas.

“Without formal registration at birth or identification documents, children may find themselves excluded from access to vital services such as education, health and social security,” the report says.

About 82 million girls aged 10-17 will be married before they are 18, losing out on education and often subjected to violence and abuse. Some 180 million children are involved in child labour, 8.4 million of them in near-slavery, prostitution and pornography, or taking part in armed conflict; 1.2 million are trafficked for sex and labour and subjected to “unspeakable abuses”, said Ann Veneman, Unicef executive director.

Governments must seek out marginalised and excluded children and bring them into school, the report says.

Karin Landgrin, Unicef’s head of child protection, said: “Education is often a protective factor against exploitation.”

Yet many governments have little data on these children and are often “in denial” about sexual exploitation.

“It takes an extra effort,” Ms Landgrin said. “If you don’t go and look for such children, they will not come to you.”

Unicef: The State of the World’s Children 2006: Excluded and Invisible; www.unicef.org