The risk algorithm, developed by a team led by Michael King and Irwin Nazareth, of University College London, was tested by 6,000 people visiting their family doctors in six countries in Europe -- Britain, Spain, Portugal, the Netherlands, Slovenia and Estonia. Its accuracy was also tested by nearly 3,000 more in Chile.

The online tool, predictD, could eventually be used by family doctors and local clinics to identify those at risk of depression for whom prevention might be most useful, the researchers said.

The study, published in the Archives of General Psychiatry, found the tool, which followed-up the participants at six and 12 months, was as accurate at predicting future episodes of depression as similar instruments developed in Europe to predict future risk of heart problems.

"Depression is a common problem throughout the world, but although we know how to treat it, we know very little about how to prevent its onset," King said in a statement.

"We have ways of predicting the onset of heart disease or stroke, but none for predicting people's risk of major depression. Our study is one of the first to develop a risk algorithm for just this purpose."