The London-based lawyer at the heart of a huge row over internet piracy, Andrew Crossley, breached the solicitors code of conduct with his method of accusing people of illegal filesharing, a judge has ruled.

ACS:Law, the law firm created by Crossley, sent tens of thousands of letters demanding "settlement" payments of about £500 from people it accused of illegal downloading.

The firm apparently recouped hundreds of thousands of pounds from the controversial "speculative invoicing" scheme before it finally brought 27 cases to court earlier this year. However, Crossley tried to halt the trial before he had to bring any evidence.

Ruling in the Patents County Court in London on Monday, Judge Birss QC described ACS:Law's pursuit of illegal filesharers as "amateurish and slipshod" and said it had "brought the legal profession into disrepute".

Birss said Crossley had breached the solicitors code of conduct because he was responsible for the licence agreement between Media CAT and the original copyright holders, and stood to profit from it. The code of conduct states that "you must not enter into an arrangement to receive a contingency fee for work done in prosecuting or defending any contentious proceedings" before the court.

The judge said: "I am quite satisfied to the standard necessary for this stage of a wasted costs application that Mr Crossley is responsible for the basic agreements [the licence agreements between Media CAT and original copyright holders] and has thereby acted in breach of the solicitors rule 2.04.

"In my judgment, the combination of Mr Crossley's revenue sharing arrangements and his service of the notices of discontinuance serves to illustrate the dangers of such a revenue sharing arrangement and has, prima facie, brought the legal profession into disrepute. It may be better placed under the revenue sharing heading in this judgment but it is, prima facie, improper conduct in any event."

Crossley is due to face the Solicitors Disciplinary Tribunal later this year. A spokeswoman for the Solicitors Regulation Authority, which finalised proceedings against Crossley in March, said on Monday: "Today's judgment supports our concerns about the effects this sort of correspondence has on the public."

Crossley could also now have to pay up to £100,000 in legal costs to those he accused of illegal filesharing. One solicitor representing five of the defendants says its bill is £90,000.

The solicitor now has about 25 days to appeal to the high court to overturn the costs order. However, Birss on Monday refused his application to appeal.

Birss said: "ACS:Law's conduct was chaotic and lamentable. Documents which plainly should have been provided [as evidence before the court] were not provided. This was not the behaviour of a solicitor advancing a normal piece of litigation."

Michael Forrester, a solicitor for the law firm Ralli which represented some of the 27 defendants accused by ACS:Law, described how they "cannot possibly have uploaded or downloaded copyright protected material".

In February, ACS:Law announced that it would no longer pursue copyright litigations after Crossley claimed death threats were causing an "immense hassle" to his family.

ACS:Law was catapulted into the spotlight in September last year when the personal details of thousands of internet users leaked from its website in the aftermath of an attack on by the hacker group Anonymous. The information commissioner is still investigating the leak, and could hand ACS:Law a £500,000 fine if it is found to have stored the details insecurely.