Depression is the leading cause of disability worldwide, the World Health Organisation (WHO) has revealed.

It estimates that the number of people living with depression globally had reached 322 million by 2015 – an 18.4 per cent increase since 2005.

The figure represents about 4.4 per cent of the world’s population.

Margaret Chan, director-general at WHO, said: “These new figures are a wake-up call for all countries to rethink their approaches to mental health and to treat it with the urgency that it deserves.”

The organisation estimates depression to cost the global economy $1 trillion annually, due to a drop in productivity and other medical conditions associated with the condition.

It says that for every dollar invested in improving access to treatment leads to a return of $4 in better health and productivity.

The international agency defines depression as “persistent sadness and a loss of interest in activities that people normally enjoy, accompanied by an inability to carry out daily activities for two weeks or more”.

Dr Shekhar Saxena, Director of the Department of Mental Health and Substance Abuse at WHO, said: “A better understanding of depression and how it can be treated, while essential, is just the beginning.

“What needs to follow is sustained scale-up of mental health services accessible to everyone, even the most remote populations in the world.”