By Sean Coughlan BBC News education correspondent

The millennium pledge made by international leaders that all children would have a primary education by 2015 is going to be “missed by a large margin”.

That’s the stark conclusion of a report published by the United Nations Educational, Scientific and Cultural Organisation (Unesco).

Despite an initial surge that saw tens of millions of extra children enrolling in primary schools, the report says progress is now “grinding to a halt”.

The report, published in Paris on Tuesday, shows the number of children without this basic level of education has fallen from 108 million to 61 million in the first decade of this century.

It means that since 2000, the percentage of the world’s children entering primary education has risen from 80% to 90%.

It’s a leap forward, but some distance from reaching the finishing line.

“It is simply unacceptable that out-of-school numbers have stagnated, and in Africa have risen,” said Gordon Brown, former UK prime minister and now UN global education envoy.

But he added: “Now is not the time for defeatism and despair,” and called on the international community to “redouble our efforts”.

Pauline Rose, director of the Education for All Global Monitoring Report, says the drive for universal primary education had enjoyed an initial “honeymoon period” with strong political backing and financial support.

If that early rate of progress had been maintained, the target would have been achieved, she says.

But at the current slow rate of change, Dr Rose says it would take at least until 2030.

Why has it proved so difficult to provide primary schools? If the world can put a spacecraft on Mars, surely it can build and staff enough classrooms. This pledge on primary schools has now been a target since 1990.

“To a large extent it’s a lack of money, aid donors have not provided the $16bn (£10bn) needed to get every child into school,” says Dr Rose.

There is also a sense of fading international attention, particularly since the financial crisis. “People have lost their interest and turned to other things,” she says.

The corrosive impact of armed conflict and political instability has also been a barrier. And the shooting in Pakistan last week of 14-year-old education campaigner, Malala Yousafza, showed the cultural barriers that remain.

There are also distinct regional patterns below the headline figures.

While many Asian countries have made strides forward, including Pakistan, there has been much less advance in sub-Saharan Africa.

The single biggest number of children out of school is now in Nigeria – with the report showing there are 3.6 million more children missing school than in 2000.

But despite the forecast of missing the target, there are some reasons for optimism.

Many more girls are in school – and in many countries there has been a substantial improvement in the availability of school places.

Ethiopia and India are given as examples of what can be achieved, with “dramatic” reductions in out-of-school children.

Tanzania trebled the proportion of national income spent on education and saw its primary enrolment rate double.

“Overall it’s a story of success. We’ve managed to make great progress, but we must not stop,” Dr Rose says.

There will also need to be a more targeted approach for those groups still missing out, she says.

The report published by Unesco shows how within countries there are deep inequalities in access to school – with the rich many times more likely to attend than their poorer compatriots.

Among wealthy families in African countries, enrolment rates are on a par with anything in the developed world.

But even the lowest level of school fees can be enough to exclude the poorest families.

It’s not only children who are missing the barest of essentials in education.

There was also a target to cut adult illiteracy by half, also by 2015, which the report predicts has little chance of being achieved – a casualty of “government and donor indifference”.

It means there are 775 million adults unable to read or write.

It’s not only individual life chances that will be lost.

The report highlights the deepening economic problem of the lack of skills needed for employment.

While there has been a focus on getting all children into primary school, in sub-Saharan Africa only 40% of youngsters stay on for secondary school.

In countries with large young populations, the combination of a tough jobs market and poor education and qualifications is a political and social tinder-box.

There are huge potential gains for countries that can raise the standard of education, says Halsey Roberts, the World Bank’s lead economist on education.

“Each additional year of schooling will increase an individual’s wages by 8% or 10% for every year that he or she works, so that obviously has a tremendous effect on your lifetime earnings,” he says. It will also bring wider benefits to families and their communities.

“If you look at the experience of East Asia, you can see that education can really transform societies.

“The best way to get across what education can mean for development is to look at an example like Korea.”

South Korea, from a starting point of great poverty, has deliberately invested in education as a route to becoming a modern industrial power.

The progress of countries such as China, Peru and Ethiopia also shows that there is nothing inevitable about the link between poverty and poor performance in education, says Andreas Schleicher, head of education at the Organisation for Economic Co-operation and Development.

He has also highlighted the curious and counter-intuitive link between a wealth in natural resources and a dearth of success in education.

This so-called “resource curse” shows that countries with very little natural resources appear to perform much better in education than those with an abundance of oil money.

The Unesco report says that if a fraction of the oil and mineral wealth in some African countries had gone to education, rather than military spending or lost to “mismanagement”, they could educate their children without any need for international aid.

But was it ever really going to be possible to reach the target of universal primary education?

Clare Short represented the UK when the promise was made at the World Education Forum in 2000 in Dakar in Senegal.

“What’s the point of a target that’s easy to achieve? That doesn’t change anything. We knew we were taking on something big,” said Ms Short.

She strongly defends the setting of the target – and says that it delivered “big, driving change”.

She also puts it into its historical context – coming in a window of optimism between end of the cold war and the beginning of the war on terror.

“The world was looking for something big to mark the millennium – the cold war was over, Nelson Mandela had been released from prison.”

The setting of such measurable goals also represented a battle of ideas in international relations.

“How do you make the world safe? Is it lots of military spending or is it more children in the world being educated.”

So what happens next?

Despite the gloomy prospects of reaching the target, the UN last month said it had secured a further $1.5bn (£0.93bn) to invest in primary education.

Gordon Brown says he wants to “concentrate the minds of governments and mobilise new resources”.

“We know it’s achievable,” said Halsey Roberts. “The question is when.”