By Jane Wakefield
Technology reporter, BBC News
File-sharing is a huge headache for copyright owners
What would you do if you received a letter accusing you of illegally downloading a hardcore gay porn movie?
For genuine pirates it may seem reasonable to take the rap on the knuckles and pay a fine of up to £500 but there are worries that innocent people are being caught in the net.
It first emerged that video games firms had joined forces with law firm Davenport Lyons to crack down on file-sharers but copyright holders of pornographic films and music tracks are also now joining the fray.
Some critics are concerned about the methodology such firms are using and question whether the evidence will stand up in court.
The main thrust of evidence used in these cases is an IP address - the unique identifying number specific to a computer, which can be obtained via file-sharing sites.
Firms must then apply for a court order requiring internet service providers (ISPs) to reveal the names and addresses associated with that IP address.
Experts question whether an IP address alone would stand up in court.
The forums of broadband site ThinkBroadband have been buzzing with users confused or angry to have received letters accusing them of illegal downloading.
"An IP address alone seems to be fairly tentative evidence," said Andrew Ferguson, editor of ThinkBroadband.
The matter gets more complicated because file-sharing content aggregators - basically the piracy equivalent of Yellow Pages which tell people what is available to download - have begun to deliberately add false IP addresses to throw copyright holders off the scent.
"The addition of false IP addresses into the trackers shows how easy it is for this to go wrong," said Mr Ferguson.
Because some of the content users are accused of downloading is pornographic, many may prefer not to pursue the matter to court.
"Proving you never downloaded or shared a file when in receipt of one of these letters is difficult. The sums of money involved are such that if a parent were to receive one they would be questioning the children, but what child admits to downloading something like XXXXX Prime Cups?" asked Mr Ferguson.
Lawyer Michael Coyle has 200 clients, many of who claim to be wrongly accused of file-sharing and said he was receiving around 50 calls a day on the issue.
Some of the calls raise issues about privacy, he said.
"One woman called me to say that having received a letter she found out her teenage son had downloaded gay porn. She was angry because she didn't know he was gay and would have preferred to have found out from him rather than from a solicitor," he said.
The Internet Service Providers Association (ISPA) , which represents UK ISPs, has seen a rise in the number of court orders being received by its members.
It is concerned that people are being wrongly targeted.
"There are lots of ways to fall through the net if you are relying on an IP address," said James Blessing, an Internet Service Providers Association (ISPA) council member.
Companies generally appoint a third party to look for file-sharers - and there are firms specifically employed to track down pirates.
Such firms use a tweaked copy of the software used by file sharers. It searches for the particular file they are targeting, whether it be a video game or a pornographic film, and checks what other IP addresses have downloaded it.
"The problem is that they copy and paste the address so occasionally they miss off the last number," said Mr Blessing.
The bigger problem is that, unlike telephone numbers which are static and unchanged, the IP addresses assigned to consumers are often dynamic.
"Sometimes the data gathered doesn't contain times and dates and as most IP addresses are assigned for a set period of time, it is impossible to identify accurately," said Mr Blessing.
In some cases people are wrongly identified because their computer is running off an unsecured wireless network, allowing other unidentified parties to steal bandwidth and download illegal content without being identified.
In a statement Davenport Lyons said the letters it sends out to alleged pirates "sets out clearly the evidence that has been collected which identifies the subscriber as the owner of the IP address used to upload our client's copyrighted material."
Wrote the company: "The steps we take on behalf of the owners of copyrighted works are an entirely legitimate attempt to protect our clients from infringement.
It added: "In a large number of cases the offer to settle is accepted because the individual identified accepts responsibility for what they have done.
A Londoner paid dearly for sharing the game Dream Pinball
In a case highlighted by consumer magazine Which, an elderly couple accused of downloading an Atari racing game said they had no wireless hub and had never even played a video game.
Atari and Davenport Lyons have since dropped the case.
The law firm said it had apologised to those who had been wrongly accused.
It added: "Where submissions are made (such as someone claiming that have not uploaded the copyright material or that the ISP has wrongly identified them as the owner of the IP address) we look into these carefully before deciding whether to proceed."
"This is the case with all legal actions where a potential defence is raised after a letter of claim is sent," it said.
BBC Radio One's Newsbeat has been contacted by several listeners who say they have been wrongly accused of downloading films distributed by German firm DigiProtect, which is also represented by Davenport Lyons.
Guidance on what evidence would stand up in court is hard to come by because, so far, no case has got that far.
Games firm Topware Interactive won an out-of-court settlement of £16,000 following legal action against Londoner Isabella Barwinska who shared a copy of the game Dream Pinball.
"It is not clear from the judgement how many games were found to have been shared or even if they were downloaded. Indeed little is clear from the judgement when so much needs to be clarified," said Victoria McEvedy, a lawyer for digital rights organisation the Open Rights Group.