Too many people are being diagnosed with depression when they are merely unhappy, a senior psychiatrist said today.

Normal emotions are sometimes being treated as mental illness because the threshold for clinical depression is too low, according to Professor Gordon Parker.

Prof Parker said depression had become a "catch-all" diagnosis, driven by clever marketing from pharmaceutical companies and leading to the burgeoning prescription of antidepressant drugs.

Writing in the British Medical Journal (BMJ), he said the drugs were being marketed beyond their "true utility" in cases in which people were unhappy rather than clinically depressed.

The psychiatrist, of the University of New South Wales, Australia, said the "over-diagnosis" of depression began in the early 80s, when the diagnostic threshold for minor mood disorders was lowered.

His 15-year study of 242 teachers found that more than three-quarters met the current criteria for depression.

Qualifying symptoms included "feeling sad, blue or down in the dumps" for two weeks, or appetite change, sleep disturbance, drop in libido and tiredness.

The psychiatrist said these symptoms were so common that most people would have them at some point in their lives. Under the current diagnosis guidelines, around one in five adults is thought to suffer depression during their lifetime.

The worldwide boom in the prescription of antidepressants in the past decade has led to criticism of drug companies' marketing campaigns.

In the late 90s, drug companies in Japan prompted recognition of "mild depression" as a condition that required medication.

GlaxoSmithKline ran an awareness campaign in the country about mild depression, which said: "Depression is a disease that anyone can get. It can be cured by medicine. Early detection is important."

Between 1998 and 2003, sales of antidepressants in Japan increase fivefold, according to the pharmaceutical industry analysts IMS Health. GlaxoSmithKline saw sales of its drug Paxil rise from $108m (£54,510,500) to $298m between 2001 and 2003.

However, mental health charities today rejected Prof Parker's assertion that depression was over-diagnosed.

Marjorie Wallace, the chief executive of SANE, said: "It is better to risk over-diagnosis than to leave depression untreated. One in 10 people with severe depression may take their own life."

Her view was supported by another study in the BMJ by Professor Ian Hickie, of Sydney University, who said the suicide rate had been reduced thanks to increased diagnosis of depression.

Ms Wallace added that depression could range from "feeling low to being so disabled that the person may be unable to get out of bed in the morning, sustain relationships or work".

However, she acknowledged that doctors were being forced to over-prescribe antidepressants for low level depression because not enough money was being put into psychological therapies such as counselling.

Dr Andrew McCulloch, the chief executive of the Mental Health Foundation said it was "very unlikely" that depression was over-diagnosed in the UK. He said it was more the case that many people did not seek help because they did not recognise the symptoms of depression.

However, he also agreed that doctors were too readily providing medication when alternative treatments could be more effective.

"Medication is relied upon heavily in the UK by GPs and patients, and is often prescribed when an alternative might have been more suitable," he said.

The World Health Organisation predicts that, by 2020, depression will be the second most serious disease globally after chronic heart disease.