American software company RealNetworks has been banned from selling a program that lets people make copies of their DVDs, in a US court decision that could have far-reaching implications.

After a year-long case over the legality of the company's RealDVD software, a district court in San Francisco ruled that Real had violated America's copyright laws and granted a preliminary injunction against Real to prevent it selling the program.

The ruling stops Real from selling RealDVD, a piece of software that allows to make back-up copies of their movie discs and save them to their computer. Although free DVD ripping software is readily available online, Real raised the hackles of Hollywood executives in 2008 because it paid for a license to the DVD Copy Control Association, believing that it could be interpreted to allow the services they wanted to provide.

In her ruling, Judge Marilyn Hall Patel said that the complex meant that it was not illegal for consumers to copy their own DVDs – just illegal to produce a program that allowed them to do so.

"While it may well be fair use for an individual consumer to store a backup copy of a personally-owned DVD on that individual's computer, a federal law has nonetheless made it illegal to manufacture or traffic in a device or tool that permits a consumer to make such copies."

The case was brought by the Motion Picture Association of America - the consortium representing Hollywood studios that has become notoriously litigious in the face of unauthorised downloading and online file sharing.

Dan Glickman, the head of the MPAA, hailed the verdict.

"We are very pleased with the court's decision," he said. "This is a victory for the creators and producers of motion pictures and television shows, and for the rule of law in our digital economy."

"Judge Patel's ruling affirms what we have known all along: Real Networks took a license to build a DVD player and instead made an illegal DVD copier."

Campaigners had argued that consumers had the right to make personal copies of material they had legally obtained, and that Real should not be punished.

Lawyers for the MPAA, on the other hand, said that Real was breaking the terms of its license and profiting from a system that made it possible for consumers to rent or borrow movies for a nominal fee before copying them.

The court sided with the film industry, largely because it said broke a licensing agreement to access privileged details of the copy protection codes, known as the content scramble system or CSS.

Although Judge Patel earlier ruling that CSS was a trade secret – indicating the possibility that she might rule against Real – another company, Kaleidascape, had recently been ruled to have not violated the law in a similar position.

Another scheme for unscrambling the code, called DeCSS, was also challenged in court after its creation - but Jon Lech Johansen, the Norwegian teenager credited with working on the system, was acquitted by a court in Oslo.

"Real will likely appeal this ruling," said Fred von Lohmann, an attorney with the campaign group the Electronic Frontier Foundation. "Whatever the outcome of that appeal, this ruling sends a chilling message to any technology innovator interested in delivering new products that interact with the DVDs you own."

Real Networks, based in Seattle, said that it was "disappointed" in the ruling and would take time to examine the verdict closely before considering whether to appeal.

"We have just received the judge's detailed ruling and are reviewing it. After we have done so fully, we'll determine our course of action," the company said in a statement.