Image copyright Unicef Image caption South Sudan has faced the destruction of classrooms

A third of the world's poorest girls, aged between 10 and 18, have never been to school, says the United Nations.

A report from Unicef, the UN's children's agency, warned that poverty and discrimination were denying an education to millions of young people.

It criticised a "crippling learning crisis" for impoverished families, particularly in sub-Saharan Africa.

The findings were published as education ministers from 120 countries gathered for a conference in London.

The Education World Forum, an annual international event, brings together representatives of education systems around the world, debating ideas about improving schools and using technology.

But the Unicef report warned that for too many of "the world's poorest children" there is no school at all.

Among children in the poorest fifth of households in the world, a third of girls have never been to school.

This was exacerbated by education budgets often being heavily skewed towards children from wealthier families, says the report.

Guinea, Central African Republic, Senegal and Cameroon were named as having the biggest imbalances, with public education spending being focused on rich rather than poorer children.

Image copyright Unicef Image caption Refugees in Chad: Conflicts have disrupted the educations of tens of millions

This unequal distribution means the poorest communities either miss out on school or are faced with large class sizes and a lack of trained teachers.

"As long as public education spending is disproportionately skewed towards children from the richest households, the poorest will have little hope of escaping poverty," said Unicef's executive director, Henrietta Fore.

Overcoming the lack of access to education for girls in developing countries has been a theme raised by Prime Minister Boris Johnson.

"I don't think that people know how stark that problem is and the damage it is doing," Mr Johnson told the BBC when he was foreign secretary.

"In countries where there is poverty, civil war, that have massive population booms, and that are prey to radicalisation, the common factor is female illiteracy, the undereducation of women and girls," he added at the time in March 2018.

Since becoming prime minister, Mr Johnson has repeated his support for an international promise to give 12 years of quality education to all girls.

Education Secretary Gavin Williamson, addressing the international education conference in London, said that Brexit would not be a "sign that we're stepping back from the world. I say quite the reverse".

He said that "our doors are open" to ideas from other countries and announced that a school exchange scheme, used by 138 schools since it was set up last year, would be extended for another year.

But the National Union of Students said that a more urgent priority should be to resolve the uncertainty over future of the Erasmus+ exchange scheme, after the UK's departure from the European Union.