The study assessed 1,000 teenagers in China with an average age of 15 years for depression and anxiety, reports the journal Archives of Paediatrics and Adolescent Medicine. It included questions such as: "How often do you feel depressed, moody or nervous when you are offline, which goes away once you are back online?"

Around 6 percent, or 62 teenagers, were classified as having moderately pathological internet use, while 0.2 percent, or two teenagers, were deemed severely at risk. Nine months later they were reassessed for depression and anxiety. More than 8 percent, or 87 teenagers, had developed depression.

The study was carried out by Lawrence Lam of the School of Medicine in Sydney, Australia, and Peng Ziwen of the Ministry of Education and Sun Yat-sen University in Guangzhou, China.

Teenagers who are addicted to the Internet are more than twice as likely to suffer from depression. For the first time, a study claims that "pathological" web use causes mental health problems.

"This result suggests that young people who are initially free of mental health problems but use the internet pathologically could develop depression as a consequence,"Mr. Lam said.