An appeals court in California has ruled that the individual behind anonymous posts on a Yahoo message board cannot be identified.

The case was bought by Lisa Krinsky, a former chief executive at drug testing firm SFBC International, over derogatory comments made on a Yahoo Finance message board.

Krinsky sued the defendant, known only as 'Doe 6' as his onscreen name contains an expletive, along with nine other posters for comments made about the company.

Although Krinsky initially won the right to have the posters identified, the new ruling stated that their right to anonymity must prevail under free speech laws.

The court acknowledged that the comments were "unquestionably offensive and demeaning", but that they came under free speech laws since they could not be considered a assertions of facts.