And campaigners fear that as computer porn becomes more widespread it will give more men unrealistic expectations of their love lives and threaten couples' relationships.

More than nine million men - almost 40 per cent of the adult male population - logged on to sex websites last year, more than four times as many as the estimated two million in 2000.

Relationship experts said women have to compete with 'perfect' models online, which could lead to an explosion of young people unable to hold down normal relationships.

The findings were made in what is thought to be the first definitive study of Internet pornography, for the Independent on Sunday by analysts Nielsen NetRatings.

And the explosion of Internet pornography has also resulted in more than half of all children viewing adult images 'while looking for something else'. Liberal Democrat health spokesman Sandra Gidley said she was 'alarmed' by the material available and particularly how easy it is for children to be exposed to it.

Men tend to prefer picture-based websites while women prefer erotic chatrooms - although they are more likely to view using porn as an act of betrayal. The UK porn industry is now worth around £1billion. The business is valued at £20billion globally.

While it appears to be killing off Britain's soft-core porn magazine market, which has halved to 500,000 sales each month since 2000, many critics believe it has opened the way to the adult movie market.

The scale of the problem has been highlighted by Operation Ore, a United States-led investigation into an online service offering thousands of images of abused children.

These research findings are really alarming. Our main concern in this part of the world is how this behaviour would fuel the spread of the HIV/AIDS.

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