Launching an international campaign to highlight the problem, WHO director general Gro Harlem Brundtland said on Friday: "The situation has even deteriorated and the burden will become even heavier in the next decade unless substantial action is taken."

Some 400 million of the world's population are estimated to have a mental illness - most are in developing countries.

The WHO believes depression will be the second leading cause of disability and health problems by 2020.

Its campaign was launched in China which officially has 16 million mental health patients, although this is thought to be a vast underestimate since only the most seriously ill are sent for treatment.

Suicide

At the launch, a Chinese official stated that some 250,000 Chinese people committed suicide every year, most of them women.

He blamed male chauvinism, poverty and overwork.

Experts said it was the first time China had officially admitted the extent of the problem.

Dr Arthur Kleinman of Harvard University, a specialist in depression, said the Chinese figures were double the US suicide rate.

China is the only country where women have a higher suicide rate than men, although it is estimated that, throughout the world, women are twice as likely to seek treatment for depression than men.

The Chinese official said the government was planning to tackle the problem by raising income, promoting sexual equality, providing more counselling services and raising awareness about mental health.

Counselling services are virtually non-existent in rural areas of the country.

The WHO campaign aims to raise awareness among health officials around the world of the extent of mental illness and their cost.

The organisation says only 35% of people suffering from depression receive treatment.

"The lack of treatment for mental health problems is appalling," said Ms Brundtland.

In India, for example, only 20% of schizophrenics and epileptics are treated, compared to 80% in the industrialised world.

The WHO wants to develop community-based mental health services which put an emphasis on family support. It says this could counter the high cost of specialist treatment.