But it is not clear whether the internet causes depression or whether depressed people are drawn to it.

Psychologists from Leeds University found what they said was "striking" evidence that some avid net users develop compulsive internet habits in which they replace real-life social interaction with online chat rooms and social networking sites.

"This study reinforces the public speculation that over-engaging in websites that serve to replace normal social function might be linked to psychological disorders like depression and addiction," the study's lead author, Catriona Morrison, wrote in the journal Psychopathology.

"This type of addictive surfing can have a serious impact on mental health."

In the first large-scale study of Western young people to look at this issue, the researchers analyzed internet use and depression levels of 1,319 Britons aged between 16 and 51.

Of these, 1.2 percent were "internet addicted", they concluded.

These "internet addicts" spent proportionately more time browsing sexually gratifying websites, online gaming sites and online communities, Morrison said. They also had a higher incidence of moderate to severe depression than normal users.

"Excessive internet use is associated with depression, but what we don't know is which comes first -- are depressed people drawn to the internet or does the internet cause depression?," Morrison said.

"What is clear is that for a small subset of people, excessive use of the internet could be a warning signal for depressive tendencies."

Morrison noted that while the 1.2 percent figure for those classed as "addicts" was small, it was larger than the incidence of gambling in Britain, which is around 0.6 percent.