The US Patent and Trademark Office has agreed to review Microsoft's software patent covering the FAT file system, in response to a request from a New York-based patent watchdog group.

In April, the Public Patent Foundation, asked the patent office to conduct a review, claiming that previous software patents had already been issued on the same technology and that Microsoft should have never been granted the patent.

"There's presumed to be harm on everyone if a patent is issued where it shouldn't be issued," said Daniel Ravicher, the foundation's executive director. "The FAT patent is causing immeasurable injury to the public by serving as a tool to enlarge Microsoft's monopoly while also preventing competition."

Talking about the USPTO's decision, he said: "We are obviously very pleased with the Patent Office's decision. This is the first step towards ending the harm being caused to the public by this patent that should have never been issued."

The patent covers a file system that has been in use since the 1970s. Microsoft was granted the patent in November 1996. The patent office has now granted the foundation's request and is beginning the process of re-examining Microsoft's patent, according to a statement from primary examiner Charles Rones dated 3 June.

In December 2003, Microsoft launched a licensing program that sought a 25 cent per unit licensing fee for devices such as memory cards and digital cameras that used the FAT technology.