Some 1,000 fresh letters have been sent to alleged illegal file-sharers
More than 150 people have approached consumer publication Which? Computing claiming to have been wrongly targeted in crackdowns on illegal file-sharing.
ACS:Law has sent thousands of letters to people claiming they have illegally downloaded material and offers them a chance to settle by paying around £500.
Which? says it has been approached by some - including a 78 year-old accused of downloading pornography - who have no knowledge of the alleged offence.
ACS:Law said its methods were accurate.
The London-based firm said that it would send more letters soon.
However, since the latest letters were sent two weeks ago, ten new people have come forward saying they have been wrongly accused.
One told Which?: "My 78 year-old father yesterday received a letter from ACS Law demanding £500 for a porn file he is alleged to have downloaded.
"He doesn't even know what file-sharing or BitTorrent is so has certainly not done this himself or given anyone else permission to use his computer to do such a thing."
Which? Computing is concerned that too many innocent people are being wrongly accused.
"Innocent consumers are being threatened with legal action for copyright infringements they not only haven't committed, but wouldn't know how to commit," said Matt Bath, technology editor of Which?
Many "will be frightened into paying up rather than facing the stress of a court battle", he added.
He advised people who believe they have been wrongly targeted to "rigorously deny it and, if possible, provide physical evidence of where they were when the infringement took place".
He also advised them to contact Which with the details of their case.
Andrew Crossley, of ACS:Law, said that some cases had been dropped although he declined to give numbers.
He said that he is convinced the method used to detect the IP address used for illegal downloads is foolproof.
"We are happy that the information we get is completely accurate," he said.
He said the letters do not accuse individuals.
"We explain that an infringement has taken place but it may not be the account holder who has done it," he said.
He advised those who believe they have been wrongly accused to seek out the "advice of a technical expert or the citizens' advice bureau".
But he warned that people "shouldn't just think that writing and saying they didn't do it will be sufficient".
Mr Crossley said the majority of illegally shared content was music with only 10% being adult content.
He told BBC News that the law firm had a range of clients that it was representing, including German content firm DigiProtect.
The company is based in Frankfurt and brands its business with the motto "turn piracy into profit".
It has represented a range of rights holders in the past including the German techno band Scooter.
It tracks down alleged pirates by logging the individual Internet Protocol, or IP, address of internet users logged on to file-sharing networks.
It then applies to the High Court to force broadband companies to release the physical contact details of customers matched to those addresses.
ACS:Law is currently under investigation by the Solicitors Regulation Authority.
Mr Crossley said that the law firm planned to send "lots more letters" this year although conceded that none of the current 10,000 actions had gone to trial. One had been pursued as a civil case and others were pending, he said.
"It has been said that we have no intention of going to court but we have no fear of it," he said.
Some are already in the process of going to court, he told BBC News, although the majority of the accused settle out of court.
Mr Bath is not convinced.
"These cases have been pending for a long time. I suspect that if they went to court it would be very difficult to proof beyond doubt that a particular individual was responsible for downloading the illegal content," he said.