Hundreds of Brits are being forced to fork out £500 after being accused of illegally downloading digital audio and pornography files.
The letters claim the recipient has illegally shared files and is subsequently required to pay a £500 fine and sign a legal undertaking agreeing not to illegally file-share in the future.
They also detail the date and time the offence occurred along with the file name.
Which? said that rights holders are identifying IP addresses that are illegally file-sharing and then getting courts to order ISPs to hand over the account identities for the IP addresses.
They are then commissioning legal firms to produce and issue the letters.
The letters, some of which are 30 pages long, are being issued even though the Digital Economy Bill, which sets out measures by the government to tackle internet piracy, has not come into force in the UK yet.
The measures suggest a 'three strikes' rule under which potential offenders will receive warning letters and emails designed to educate them on their wrong-doing. Should they continue to offend, web users will then face disconnection from the web.
However, the letters that have been received so far appear to be less about educating web users and more about issuing a penalty.
They certainly don't offer any information or advice on what consumers should do if they were not the one using the connection to illegally download.
Which? was highlighting the issue at the 'Principles and Practicalities of Copyright' event, hosted by TalkTalk in a bid to educate MPs and peers about the damaging effect of the Digital Economy Bill on human and consumer rights.
According to Matthew Bath from Which? many recipients are choosing to pay the fine rather than fight them in court because they're unsure of how to prove they are innocent.
Others, meanwhile, are concerned that fighting the accusation could affect other areas of their lives such as their employment or even their marriages, in the case of those accused of downloading pornography.
Which? is urging the government to implement a process that will stop law firms issuing these letters and also consider creating a place that Brits can turn to for advice and help in fighting the accusations as well to discover whether they did actually infringe copyright law.
Bath said education was a key factor in tackling internet piracy.
According to Which? a number of consumers issued with letters have turned towards the Citizen's Advice Bureau (CAB) for help. However, the CAB has been unable to offer advice, leaving consumers out in the cold.
Andrew Crossley, founder of ACS Law said the firm began working with a number of copyright holders in May last year and sent out approximately 6,000 letters notifying individuals their web connection had been used for illegal file-sharing.
However, he said the letters were just "six pages long with evidence, a court order and a payment/undertaking form attached".
In November, more letters were sent, although Crossley said this was less than 500, with less than 1,000 more being sent since.
"It is incorrect to state that people are 'fined'. We invite people to enter into a compromise agreement in order to avoid what could otherwise prove to be costly litigation," said Crossley
Crossley said the firm provides details on its website of how the matter should be dealt with, along with a guide to evidence and a number of frequently asked questions.
"We also state very clearly that if in doubt the recipient of the letter should seek legal advice from a solicitor or Citizen’s Advice Bureau."
ACS said the information gathered by various data monitoring companies on behalf of the copyright holder was "accurate" and the data monitoring companies' methods and procedures have been analysed and scrutinised by independent experts.
"Consequently, any attempt to avoid or deny liability has to be very compelling for my clients to instruct us to close a case," he added.
"In my opinion and that of my clients, it is insufficient for people to say in relation to the identified infringement that 'it did not happen', as we have evidence that it did."
Crossley also said it was insufficient to blame it on a child "as the parent should take responsibility for the actions of children under their care".
"File sharing is illegal, it costs this country billions of pounds in lost sales to media-providing industries. Our clients are entitled to pursue instances of identified infringement with all vigour and determination and my firm has been retained to help them achieve that," he added.
Crossley's full response can be seen here.
TalkTalk has been campaigning for some time against the proposals laid out the Digital Economy Bill.
The ISP is concerned many Brits will find themselves disconnected from the web because their Wi-Fi connection has been hijacked by cybercriminals.
According to chief executive Charles Dunstone, web users that develop tools which can be used to access content illegally are often not motivated by money. Instead they are irritated by digital rights management (DRM) restrictions placed on content by labels and studios.
"Most fans grudgingly put up with it but some are smart enough to develop applications which allow content to be copied from one format or device to another," he said.
"If the Digital Economy Bill becomes law, more tools will emerge and they will be simple enough for anyone to use," he added.