Google has been cleared to show thumbnails from pornography sites in its search results.

A San Francisco appeals court reversed an earlier injunction awarded to adult website Perfect 10, which claimed that Google's search thumbnails violated its copyright.

The court found that Google's use of the images was 'fair use' under existing law.

"We conclude that Perfect 10 is unlikely to be able to overcome Google's fair use defence and, accordingly, we vacate the preliminary injunction regarding Google's use of thumbnail images," Judge Sandra Ikuta wrote for a three-judge panel.

"We conclude that the significantly transformative nature of Google's search engine, particularly in light of its public benefit, outweighs Google's superseding and commercial uses of the thumbnails in this case."

However, Google's 'view full sized image' function may be unlawful, the court found, advising that a lower court should rule on this.

Perfect 10, which charges $25.50 for access to its site, first complained to Google in 2001 and filed a case against the company in 2004. A year later it filed a similar case against Amazon.

The Motion Picture Association of America and a number of photographic agencies made legal filings in support of Perfect 10.