These disorders cost the global economy $1 trillion a year, according to a study led by the WHO which estimates for the first time both the health and the economic benefits of spending more on treating the most common forms of mental illness.
Common mental disorders such as anxiety and depression are increasing worldwide, and the number of people suffering from them rose to 615 million in 2013 from 416 million in 1990, the U.N. agency said in the study.
Last year, world leaders included mental health in an ambitious plan to end poverty and inequality by 2030, enshrined in 17 Sustainable Development Goals.
"We must now find ways to make sure that access to mental health services becomes a reality for all men, women and children, wherever they live," she added.
The study, published in the same week as the World Bank and the WHO hold a high-level meeting on mental health, is based on data from 36 low, middle and high-income countries.
Governments spend on average 3 percent of their health budget on mental health, ranging from less than 1 percent in low-income countries to 5 percent in high-income countries, according to the WHO's survey Mental Health Atlas 2014.
"Despite hundreds of millions of people around the world living with mental disorders, mental health has remained in the shadows," said Jim Yong Kim, president of the World Bank Group.
"This is not just a public health issue - it's a development issue. We need to act now because the lost productivity is something the global economy simply cannot afford," he added.
The World Bank/WHO meeting takes place on Wednesday, during the World Bank/International Monetary Fund Spring Meetings also taking place in Washington.
It brings together ministers of health and finance, donors and multilateral organizations, and aims to move mental health to the mainstream of the global development agenda from the margins.
"Mental health needs to be a global humanitarian and development priority - and a priority in every country," said Arthur Kleinman, professor of medical anthropology and psychiatry at Harvard University and an expert on global mental health.
"We need to provide treatment, now, to those who need it most, and in the communities where they live. Until we do, mental illness will continue to eclipse the potential of people and economies," he added.
(Reporting by Alex Whiting, editing by Tim Pearce.; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, corruption and climate change. Visit news.trust.org) via Reuters