About 20 per cent of players are women, drawn by the social an cooperative element of the games.

"Although I think genuine addiction is fairly low, the thing about online gaming is that the game never stops," said Prof Griffiths. "With a stand alone game you can switch it off and come back the next day, but with an on-line game it's very difficult to log off when you know half the world has just logged on.

"Many gamers play excessively and display few negative consequences. There is nothing wrong in itself with doing something excessively, unlike gambling gaming has little or no financial consequence.

"However, the 24-hour a day never-ending online games may provide a potentially addictive medium for those with a predisposition for excessive game playing."

The research is to be published in an upcoming issue of the journal CyberPsychology and Behaviour.

EverQuest and World of Warcraft are two of the biggest online games.

Seven million people subscribe to World of Warcraft to join a virtual world of Lord of the Rings-type characters such as orcs, elves, gnomes, dwarves and humans. Fees are typically about ¡ê8 a month.

But such games have come in for much greater criticism.

A woman in the U.S blamed a game on the death of her 21-year-old son, who had a history of mental health problems, and shot himself while still sitting at his computer.

In countries such as the US, China and South Korea it is not unusual to find a whole host of special treatments and clinics claiming to hold a cure for the compulsive gamer.

In South Korea, the authorities have become so concerned by the numbers of young people spending too much time on computers that the government launched a nationwide addiction hotline to help combat the problem.

In January this year a Dutch addiction centre added Europe's first rehab clinic for cyber-addicts, after an increase in gamers yearning to cure themselves of their online vices.

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