It claims that people are spending an increasing amount of time online and then lie about it to hide their activities.

In a paper to be published in the student edition of the British Medical Journal, Dr Kimberly Young, director of the centre, sets out a series of questions she says determine whether or not someone is an Internet addict.

However, other psychologists working in the same field say that her criteria are too wide ranging to provide a useful definition.

Self-test reveals all

Dr Young said that an Internet addict would answer yes to at least five of the following questions:

• Do you feel preoccupied with the Internet (you think about your previous online activity or anticipate your next session)?
• Do you feel the need to use the Internet with increasing amounts of time in order to achieve satisfaction?
• Have you repeatedly made unsuccessful efforts to control, cut back or stop Internet use?
• Do you stay online longer than originally intended?
• Have you jeopardised or risked the loss of a significant relationship, job, educational or career opportunity because of the Internet?
• Have you lied to family members, therapist or others to conceal the extent of involvement with the Internet?
• Do you use the Internet as a way of escaping from problems or of relieving, for example, feelings of helplessness, guilt, anxiety or depression?

"Most people agree that the Internet is a productive tool, but research findings document serious negative consequences when it is used in a negative manner."

The anonymity of the Internet made it easier to indulge the addiction, and encouraged "deviant, deceptive and even criminal online acts, such as the development of aggressive online personas or the viewing and downloading of illegal images".

Dr Young - who offers E-mail consultations at $15 a time - said treatment should focus on getting an individual to regulate and moderate Internet use.

'Devastating effect'

Dr Mark Griffiths, a psychologist at Nottingham Trent University, has studied the effects of excessive Internet use and is familiar with Dr Young's work.

He said that, in its favour, it raised the profile of an important contemporary problem.

"Most Internet addicts are young, socially unskilled men," he told BBC News Online. "It's tragic - these people have the same problems as any other addict."

However, he said Internet addiction was a small problem at present, but Dr Young's criteria classified too many people as addicted to the Internet.

"A lot of these people aren't addicted to the Internet - they're addicted to sex or gambling and they use the Internet as a tool," he said.

"You can't classify an addiction in terms of its medium - if someone's addicted to gambling and spends all their time in a betting shop, we don't say they're addicted to betting shops."