Rates of depression are rising
The World Health Organization predicts that within 20 years more people will be affected by depression than any other health problem.
According to the WHO, depression will be the biggest health burden on society both economically and sociologically.
Yet, it says most developing countries spend less than 2% of their national budgets on mental healthcare.
The warning comes as the first Global Mental Health Summit starts in Athens, Greece.
In 2030 this will be the single biggest cause for burden out of all health conditions
WHO figures reveal that currently, over 450 million people are directly affected by mental disorders or disabilities, most of whom live in developing countries.
The five-day summit in Athens will provide the opportunity to address what the organisers are calling a crisis in global mental healthcare.
"WHO figures clearly show that the burden because of depression is likely to increase - so much so that in 2030 this will be the single biggest cause for burden out of all health conditions," Dr Shekhar Saxena of the Department of Mental Health at the WHO, told the BBC World Service.
The scientific concept of "burden" is the measure of years lost of life, due to early death or severe disability brought on by a certain illness, in this case depression.
Dr Saxena says depression is much more common than some other diseases that are more widely feared such as HIV-Aids or cancer.
"One could call it a silent epidemic because depression is more often being recognised, but it has been there throughout and is likely to increase in terms of proportion when other diseases are actually going down."
The increasing burden will be a particular problem for developing countries because they have fewer resources to allocate to mental health.
THE SILENT EPIDEMIC
About half of mental disorders begin before the age of 14
Around 20% of the world's children and adolescents are estimated to have mental disorders or problems
Most low- and middle-income countries have only one child psychiatrist for every 1 to 4 million people
About 800,000 people commit suicide every year, 86% of them in low- and middle-income countries
More than half of the people who kill themselves are aged between 15 and 44
The highest suicide rates are found among men in eastern European countries
"We have figures to show that poorer countries have actually more depression compared to richer countries and even poor people in rich countries have a high incidence of depression compared to the richer people in the same countries," says Dr Saxena.
Yet high-income countries allocate 200 times more resources to mental health than low-income ones.
It accounts not only for a significant proportion of government spending in developed countries, it also makes a impact on their GDP as well.
Professor Martin Prince, professor of epidemiological psychiatry at King's College, London has tried to calculate in financial terms how much of a burden a depressed person can become.
"Part of this is through lost productivity because people with serious depression are much less likely to be employed and to stay employed. Then there's the cost to society of providing, for example, incapacity and unemployment benefits, particularly in rich developed countries," he says.
"These costs combined amount in the UK, it's estimated, to about £12bn ($19bn) per year or around 1% of the gross national product, so these are absolutely enormous sums."
With the expectation that the burden from mental illness is going up and will continue to increase in coming years, Dr Saxena says societal attitudes towards mental illness need to change.
"Depression is as much of a disease as any other physical disease that people suffer from and they have a right to get correct advice and treatment with in the same health care settings which look after other health conditions."