Nearly half the world's schools lack clean drinking water, toilets and handwashing facilities, putting millions of children at risk of disease, experts warned on Monday.

Almost 900 million children have to contend with a lack of basic hygiene facilities during their education, putting their health at risk and meaning some have to miss school.

"You can't have a quality learning environment without these basics," said Dr. Rick Johnston of the World Health Organization (WHO), a lead researcher on the project.

World leaders have signed up to global pledges to provide safe water and hygiene facilities for all and ensure every child gets a comprehensive education by 2030 under the United Nation's sustainable development goals.

A lack of safe water and sanitation facilities can cause dehydration, illness and even death.

But many children are forced to risk their health to take part in classes, according to the report produced in conjunction with the United Nations Children's Fund (UNICEF) and the WHO, the first to look specifically at provision in schools.

It found nearly a third of primary and secondary schools lacked a safe and reliable drinking water supply, affecting nearly 570 million children. Nearly 20 percent of schools had no safe drinking water at all.

Just over a third of schools lacked adequate toilet facilities, affecting more than 620 million children. Almost one in five primary schools and one in eight secondary schools were considered to have no sanitation.

Nearly half lacked proper handwashing facilities, essential for helping prevent the spread of infections and disease. Nearly 900 million children were affected, the report found.

Sub-Saharan Africa, East and Southeast Asia had some of the worst facilities.