The aim in Doom, the top-selling computer game on PCs at the moment, as in so many other computer games, is to kill as many aliens and zombies as possible. What makes it different is that it seems startlingly realistic and gruesome. Instead of zapping aliens with a laser gun or bombing them from an aircraft, the player of Doom can use a chainsaw or his bare fists as well as the usual armoury of futuristic weapons.

The makers, three young Texans, drew on imagery from horror films like The Texas Chain Saw Massacre in their work. The monsters burst open with a squelching noise when hit, spattering blood around the screen. The action takes place in a succession of mazes decorated with rotting impaled corpses and pyramids of skulls. It is full of nasty little touches like pools of poison slime and barrels of radioactive waste. The player's view is three-dimensional, from behind the helmet of the character he plays, so that he seems to be drawn into the maze himself.

Small children can be terrified by these things. Mine certainly have been. When my son was eight he developed a fascination for one of Doom's predecessors, a game called Dungeon Master, which seems now startlingly primitive but was the first computer game to place the player in this kind of three-dimensional perspective in a maze. My son would insist that I play it, then hide behind the sofa as I did so. My daughter found Doom so disturbing that we took it off the computer at home.

I imagine that far more gruesome things are to be found in horror videos, though I have no intention of finding out. But computer games have a dimension of interactivity which makes them much more gripping. No one ever rewound a horror movie again and again to watch it until three or four in the morning, as people in the grip of a game like Doom will do.

Computers are the fastest growing section of the entertainment industry. The computer games business is worth pounds 700m a year in the UK alone.

What is worrying the respectable firms of the industry, grouped in a body called Elspa, is that all human life will shortly be available on computer disk. Elspa held a press conference at the European Computer Trade Show in Islington, north London, yesterday to appeal for more and better-trained policemen to deal with the twin problems of software piracy and software pornography.

Elspa is proud of its system of self-censorship, under which computer games are rated like videos for the amount of nudity and violence they contain, and then graded for suitability for adolescents.

However, Doom is not rated under the Elspa system at all. It has escaped the system because it is distributed across frontiers down telephone lines. The American publishers made it available to anyone on the global computer network known as Internet, which links at least 30 million people around the world, perhaps 80,000 of them in Britain. It is sold as 'shareware': the first part of the game is free to anyone with a modem. Admittedly it takes half an hour or more to download it, but that is the only cost to the player. Once downloaded it can be copied on to floppies and freely, legally distributed to people who have no modems. This is what many games players treat the whole industry as producing: and such widespread theft of software was what led to Elspa being set up in the first place.

However, Doom became a world-wide craze quite legally and with the blessing of its makers. They made their fortune from players who wanted more: for dollars 50 they sell a full version of the game, three times as large as the free one - and, incidentally, far too large to distribute practically down telephone lines.

An intelligent, determined and computer-literate child with a credit card and a flexible phone bill could get Doom and, with more difficulty, amass a huge collection of startlingly disgusting and realistic pornography.

A modern personal computer with a CD-rom drive can reproduce moving images far more vividly (though on a much smaller scale) than a video recorder can. The back pages of American computer magazines contain numerous advertisements for 'Adult Bulletin Boards' which contain huge numbers of pornographic files. These can be pictures, usually scanned from magazines or 'grabbed' from videos, moving full-colour sequences, some with noises, or simply stories. All can be downloaded by anyone with a modem and a credit card.

John Loader, Elspa's chief investigator, claimed yesterday that children did not even need money to get at this pornography. There is a network of hackers, he said, 'young people of any age under 20' who were exchanging tips about how to dial the US free of charge and then download pornography for hours, which they then passed around their friends.

However, all the evidence suggests that even in the States, where the use of computers and modems is far more widespread than in this country, partly because local phone calls are free, computers are still not a very important channel for the distribution of pornography. What they do distribute, and very successfully, is violence.

The next version of Doom will be sold by more conventional means, and into a ready-made market, when it appears in October. It will even have been certified by the British Video Standards Council. But the success of Doom 1 indicates the enormous difficulty facing any country which wishes to censor computer games. It is almost as difficult to keep unauthorised computer software out of a country as it is to keep out unauthorised radio broadcasts.

No one knows how many computers in Britain are hooked up to to the various international computer networks which are converging into a single global Internet. Compuserve, the largest commercial networking organisation in Europe, has been signing up 1,000 new English customers a week this summer to join the 2 million members it has worldwide.

Compuserve does not distribute pornography, and would not distribute Doom's immediate predecessor, Wolfenstein, because the plot involved shooting Nazis in a castle hung with swastikas, which violated German laws against the display of Nazi insignia. It does, however, carry the global discussion groups known as Usenet. If you search through Compuserve for Usenet discussions about sex, you will find nothing. But if you know what to ask for, you will find a great deal.

One Usenet group is devoted entirely to the sale of pornography. It is not usually computer porn: most of it seems to be videos. All of it must be legal where it is physically offered for sale. An American with the nom de plume 'ak47yberverse. com' offers 'about six videos of myself having sex with various chicks to trade. I'm looking for people out there who have HOMEADE (sic) footage to trade.'

An Englishman named Mark Aldrich appeals for 'someone in the Netherlands who will send me X-rated videos direct'. And so on. At the moment all this is a very small trade. It may never grow much bigger, at least until our present telephone lines are replaced with something much more capacious. Computer networks at the moment are like the News of the World in that all human life is there, and some of it doing unsavoury business. The information superhighway will not change the world by globalising pornography, though it will do that, if only as a side-effect. It will change the world by making all commerce even more global. Anything legal anywhere in the world will be easily brought anywhere else. All this is of course dependent on the most important computer networks of all: the ones that banks and credit card companies use to shift money round the world at the speed of light.

In the great world-wide souk that is coming, everything will be for sale somewhere; and if money can be made out of it, it will be respectable somewhere. Against this we have no defences.

(Photograph omitted)