London: It's now official: Coffee is good for you, as its mood-boosting effect could well be lasting far longer than we realise, says a new Harvard University study.

The study, which compared the coffee intake and risk of depression amongst nearly 51,000 women over 10 years, found that those who drink four or more cups a day are a fifth less likely to become depressed.

And those who drink between two or three cups a day cut their risk by 15 percent, found the study, published in the Journal of American Medicine Association.

Though the study did not look at men, other studies have already found that it has a similar effect.

The Harvard scientists believe that caffeine works like antidepressant pills by stopping the production of certain hormones such as serotonin.

They also said it improves our feelings of well-being and gives us more energy over long periods.

"Our results support a possible protective effect of caffeine, mainly from coffee consumption, on risk of depression," study author Dr Michel Lucas, from the Harvard School of Public Health in Boston, was quoted as saying by newspaper.

"Further investigations are needed to confirm this finding and to determine whether usual caffeinated coffee consumption may contribute to prevention or treatment of depression," he added.

The researchers also asked the women how much tea they drank, chocolate bars they ate each day, their alcohol consumption and whether they did exercise.

It was found that coffee had the strongest influence reducing depression, but the decaf variety didn't work at all.

The scientists think that in future coffee could be used as an antidepressant or as a means of preventing depression.

The researchers pointed out that caffeine had "well-known psychostimulant effects' including 'increased sensations of well-being and energy".

In future coffee could be used as an antidepressant or as a means of preventing depression, they said.

Last year, a study in Finland discovered that men who drank more than four cups a day were also far less likely to become depressed.

Women are far more likely to develop depression than men and figures show that one in seven will be affected at some point in their lives.

And last month German researchers claimed that women are now twice as likely to suffer from depression compared with 40 years ago because they are trying to juggle families and careers.

It's already known that coffee can help stave off a range of illnesses including prostate cancer, gout and Alzheimer's and may even boost brain power.

But it isn't entirely beneficial. Having too much can also raise the blood pressure and increase the heart rate, the researchers warned.

And pregnant women are advised to drink no more than two cups a day to reduce the likelihood of a miscarriage or their babies being underweight.