Thousands of computer users who breach copyright by sharing games and films are facing legal action from producers determined to protect their interests.

Topware Interactive, which produces the computer game Dream Pinball 3D, obtained default judgments in the Central London County Court against four individual file-sharers on Friday. Each was ordered to pay the company interim damages of £750 by the end of next week. The final figure could be as high as £2000 per person, plus costs in the region of £1500.

Why so much - when a licensed copy of the game costs as little as £16? It's because the game may be downloaded many times from a single computer and damages are intended to reflect the claimant's losses.

Topware's Mayfair solicitors, Davenport Lyons, say they will issue a " substantial number" of new claims this week on behalf of several clients. Their targets are people who share music, films and games through peer-to-peer networks. These give subscribers direct access to each other's computers, allowing people to download files without paying fees to the copyright owner.

The problem, of course, is finding out who the file-sharers are. All users must subscribe to an internet service provider - an ISP. But the ISPs' trade association claims that breach of copyright is nothing to do with them.

"ISPs bear no legal liability for illegal file-sharing as the content is not hosted on their servers," the association says. "They are 'mere conduits' of information, no more able to inspect and filter every packet passed over their network than the Post Office is to open every envelope."

Well, maybe. ISPs move swiftly enough to block child pornography, for example. And what they do have is the name and address of every subscriber.

While online, each internet user is allocated a unique number - known as an IP address. Computer experts can search for file-sharers who have copyright material on their computers and log their IP addresses. But only the ISPs can say which customer was using a particular IP address at a particular time.

ISPs will sometimes warn customers if their accounts are being used for illegal downloads - as Virgin Media agreed do last month, following a request from the music industry. But Davenport Lyons could not write solicitors' letters to hundreds of people whose computers had been used for illegal game-sharing until the High Court had ordered their identification. Those who failed to provide a satisfactory response can now expect to be sued.

With the growth of television-ondemand, TV and movie producers have even more to lose from the illegal sharing of downloaded video files. But the big producers have been reluctant to take legal action, perhaps for fear that it would rebound against them. Instead, they are hoping to persuade ISPs that cracking down is in the interests of providers too - not least because illegal file-sharers take up so much of the ISPs' limited bandwith.

NBC Universal, which owns the US television network and the Hollywood studios, is so concerned that it sent its executive vice president and general counsel to London last week.

Rick Cotton has managed to raise illegal file-sharing at the highest level with each of the eight major ISPs in the US. But he still has a way to go here.

"I would say that the UK ISPs are a bit earlier in the evolution of their thinking," he tells me.

Getting something for nothing may seem attractive to consumers - but Cotton insists they lose out in the end.

"Most consumers would rather have high-quality legitimate services at reasonable prices without opening their computers up to millions of other people and risking the safety of their personal information," he says. "Why should they want a minority to free-ride on the network at their expense?"

WHAT THE RULING MEANS:

27 June court order: Judgment for claimant, Topware Interactive
• Defendants to pay £750 by 4pm on 11 July
• Defendants must also pay claimants' costs to date
• Inquiry as to further damages payable by defendants
• Claimant to serve witness statement and evidence regarding damages