Experts at a World Health Organization conference in London on Thursday heard that part of the problem was that people, particularly men, masked their symptoms.

Another reason is that because of the stigma attached to mental illness, depressed patients are more likely to go to their GP with physical symptoms such as chest pain or insomnia.

Professor Simon Wessely, of the Institute of Psychiatry in London, said doctors were more likely to accept physical symptoms as a "real" illness.

He said: "They are not all in the mind. Depression is always associated with physical symptoms," he said.

Social changes, including increasing job insecurity, worries about the environment and the rising elderly population were blamed for the increase in depression.

The problem is likely to have a huge economic impact, including work days lost. The conference was told 27% of days off work were a result of mental illness.

Mass confusion

But Dr Jo Asvall, the WHO's regional director for Europe, said treatment was improving.

However, there were problems identifying which treatments worked best.

He said: "There is no country that knows what it is doing in healthcare.

"Doctors, psychiatrists and other healthcare professionals are working blindfolded."

Trials based on patient outcomes had begun, but research was just in the early stages in most places, he said.

Doctors complained that patients were being confused about what depression was because of commercial interests.

Professor Edward Shorter, of the University of Toronto, Canada, said some drug companies were promoting anti-depressants as a cure for unhappiness, confusing unhappiness with depression - a defined disorder with specific symptoms.

Dr Bedirhan Ustun, of the WHO, said studies showed effective treatment of depression could be more cost effective than treating HIV or leukaemia.

The problem was that many people did not seek help, especially men.

The conference heard how men tend to mask their depression by drinking or adopting aggressive behaviour.

Professor Per Bech, a WHO expert, said men showed different symptoms of depression than women - who are twice as likely to be treated for the condition - such as self pity and social anxiety.

Women were more likely to admit to a depressed mood or crying.

Doctors needed to be more aware of these differences, the conference heard, so they can intervene in the early stages.

Baby blues

Experts also highlighted the fact that, while women suffered depression linked with pregnancy and childbirth, men could also suffer from "baby blues".

Professor Bech said a Danish study showed the birth of a first child was the most common reason for divorce.

The conference also heard how elderly people run the highest risk of suicide.

Professor David Ames, a geriatric psychiatrist from Australia, said this was partly due to ageism and the fact that elderly people are more likely to deny they have mental health problems.

He said people living in institutions like care homes were more than twice as likely to suffer from depression than those living at home.

He said factors affecting depression in the elderly fear of loss of independence and dignity and social isolation.