Bloggers who posted links to a software key that would unlock the copy protection on some high-definition DVDs have been threatened with legal action.

The entertainment industry-backed consortium which developed the protection said that it was looking at “technical and legal tools” to confront bloggers who made the key available, saying they had “crossed the line.”

The 32-digit key appeared widely on the internet this week, including in a YouTube video and on T-shirts, and many bloggers considered publishing it as an exercise in free speech, one alluding to Thomas Jefferson’s Declaration of Independence.

Michael Ayers, chair of the group that built the software, which uses digital rights management (DRM) technology, said that there was “no intent from us to interfere with people's right to discuss copy protection,” and that the body had received "good cooperation from most folk" in preventing the leak of the key.

"Some people clearly think it's a First Amendment issue. We respect free speech – we know some people are critical of the technology,” Mr Ayers told the BBC. "But a line is crossed when we start seeing keys being distributed and tools for circumvention. You step outside of the realm of protected free speech then."

Earlier this week a row broke out when Digg, a popular technology news site, took down links to the key, saying that it had received ‘ cease and desist’ order from the group which developed the protection, known as AACS.

Facing open revolt from readers angry that the site had deferred to the owners of Advanced Access Content System (AACS), the site’s founder, Kevin Rose, reposted the link to the key, saying that he would “deal with the consequences.”

Mr Ayers would not comment specifically on the AACS group’s plans, but said it would take “whatever action is appropriate. We hope the public respects our position and complies with applicable laws.”

He said that tracking down those who had published the key was a "resource-intensive exercise".

According to a Google search, almost 700,000 pages have published the key.

Jonathan Zittrain, a professor of internet law at Harvard Law School, said that assuming the key could break a DVD, it's distribution would infringe the provisions of the Digital Millenium Copyright Act (DCMA).

"If a site isn't policing the distribution of the code, and a publisher has issued a takedown notice, which is then not obeyed, they would likely reserve their right to sue," he said.

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