One in four young people in developing countries are unable to read a sentence, according to a report, which warns that poor quality education has left a "legacy of illiteracy" more widespread than previously believed.
Research published on Wednesday by Unesco, the UN's educational, scientific and cultural body, suggests that 175 million young people lack even basic literacy skills.
"Access [to education] is not the only crisis – poor quality is holding back learning even for those who make it to school," said Unesco director-general, Irina Bokova, in a foreword to the 11th annual Education for All global monitoring report, which measures progress towards global goals.
An estimated 250 million children are not learning basic reading and maths skills, according to the report, even though half of them have spent at least four years in school. This "global learning crisis" costs developing countries billions of dollars a year in wasted education funding, it warns.
Adult literacy has remained stubbornly high over the past decade, the report adds. In 2011 there were 774 million illiterate adults, a decline of 1% since 2000. This figure is projected to fall only slightly, to 743 million, by 2015.
Ten countries – India, China, Pakistan, Bangladesh, Nigeria, Ethiopia, Egypt, Brazil, Indonesia and the Democratic Republic of the Congo – account for almost three-quarters of the world's illiterate adults, according to the report.
Globally, almost two-thirds of illiterate adults are women, a figure that has remained almost static since 1990. If current trends continue, the poorest young women in developing countries are not expected to achieve universal literacy until 2072.
Pauline Rose, the report's director, said literacy and adult education have suffered from relative neglect, as attention has focused on boosting primary school attendance rates in poor countries. Measures of progress at the national and global level often obscure deep inequalities, she added.
If current trends continue, for example, the wealthiest boys in sub-Saharan Africa will achieve universal primary completion in 2021, while the poorest girls will have to wait until 2086.
Rich countries' education systems are also failing minorities, says the report. In New Zealand, almost all rich students achieve minimum standards in grades four and eight, while only two-thirds of poor students do. More than 10% of grade eight students in Norway and England performed below minimum learning levels in mathematics in 2011.
Last year a survey of basic skills by the Organisation for Economic Co-operation and Development sparked widespread debate about illiteracy and innumeracy among young people and adults in rich countries. A failure to sustain post-16 education and deep-rooted problems of poverty and social inequality were blamed by some commentators for England's poor showing in the rankings.
The NGO Save the Children said young people from low-income families in the UK were already falling behind at school by the age of seven, with most unlikely to go on and achieve good grades in maths and English.
The report says governments must rethink their teaching policies and redouble efforts to ensure marginalised and disadvantaged learners benefit. Governments must train teachers to support the weakest learners, as well as provide incentives to attract and retain the best instructors, it says.
Many developing countries have rapidly increased their teacher numbers by hiring people without training. This may help get more children into school but it puts education quality in jeopardy, warns the report. "What's the point in an education if children emerge after years in school without the skills they need?" said Rose.
David Archer, head of programme development at the NGO ActionAid, praised the report's attention to adult literacy and education. "Post-independence most African governments ran adult literacy campaigns to redress historic injustices. Today there is an urgent need for new long-term investment in adult literacy programmes in order to redress the inequalities and injustices that arise from failing education systems," he said.
Improvements will come at a cost, says the report, which estimates that basic education alone is underfunded by $26bn (£43m) a year.
Countries should commit to spending at least 6% of their gross national product on education, the report says. Donors must increase their aid for education and sharpen its focus to support the poorest and most underserved populations. It stresses that countries cannot afford to keep adult literacy and education off the agenda.
The 2000 World Education Forum summit in Dakar, Senegal, agreed six global goals to meet the learning needs of all children, young people and adults by 2015.
According to the Unesco report, no country is projected to reach all the goals – which cover early childhood care, primary and secondary schooling, adult literacy, gender equality and educational quality – by the deadline.