ACS:Law, which courted controversy when sending letters to alleged filesharers, has shut down.
No precise reason has been given for the closure of the firm, which had been criticised from various corners, including music industry representative body BPI and operator O2.
The law firm sent "speculative invoicing" letters asking the supposed copyright infringers to pay up a one-off penalty or face going to court.
ACS:Law has been involved in a court case in which it was representing pornography licensee MediaCAT a company that has also reportedly shut down as well, according to a Torrent Freak report.
Last month, the law firm chose to stop sending letters during the court case after Andrew Crossley, head of ACS:Law, said he had received death threats, and had his email hacked.
MediaCAT had attempted to pull out of the case altogether, but Judge Birss QC told the court it would not be so simple to stop the case, partly because the actual copyright holders were not present for the hearing.
Now, lawyers for the defendants want the case to continue so damages can be claimed.
The case is due to be heard at the Patent County Courts on Tuesday afternoon
Meanwhile, Crossley faces an investigation from the Solicitor's Regulation Authority (SRA).
The SRA had been contacted by Which? after the consumer protection group took umbrage with ACS:Law's actions.
Peter Bradwell, a copyright specialist from the Open Rights Group, said it would be a shame if ACS:Law manage to avoid accountability for any wrongdoing "by disappearing in a puff of smoke like some cartoon villains."
"But the good news here is that the courts are doing the right thing by scrutinising them so closely. It hopefully will make it less likely anybody can get away with these miserable schemes in the future."
Deborah Prince, Which?'s head of legal, said she found it difficult to sympathise with the fall of ACS:Law.
"These pay up or else' letters have caused incalculable stress to many people," Prince said.
"We hope that the judge in tomorrow's court case will clarify the law to ensure that people cannot be persecuted as illegal filesharers simply for owning an IP address."
