Girls are more likely to miss school than boys

According to its estimates 100 million of primary school age are missing out, compared with 115 million in 2001.

However, Unicef said its target of all children being able to complete a primary education by 2015 would not be met unless more girls attended.

In 55 out of 180 countries looked at, there was little progress toward this.

'Not just learning'

For every 100 boys out of school there were 117 girls, Unicef said.

The Middle East and north Africa, south Asia, and western and central Africa were not on course for gender balance.

Unicef executive director Carol Bellamy said: "Education is about more than just learning. In many countries it's a life-saver, especially where girls are concerned.

A quantum leap is needed both to break down the barriers keeping girls out of school Carol Bellamy, Unicef

"A girl out of school is more likely to fall prey to HIV/Aids and less able to raise a healthy family."

Poverty was another "fundamental barrier" to education.

According to Unicef, children from the poorest 20% of households in the developing world are three times less likely to go to primary school than those from the wealthiest 20%.

Meanwhile, 75% of children out of education in these countries had mothers who had not gone to school.

Ms Bellamy said: "This report proves that our strategic focus on getting more girls into school is working to increase attendance rates for boys and girls in primary school.

"But it also makes clear that a quantum leap is needed both to break down the barriers keeping girls out of school and to make school available to all children."