A tiny New York-based non-profit patent watchdog group is taking on Microsoft by claiming that a seven-year-old patent Microsoft holds for the FAT file system is invalid and should be revoked.

In a formal request sent last week to the US Patent and Trademark Office in Washington, Daniel Ravicher, executive director of the Public Patent Foundation, asked the patent office to conduct a review because previous patents had already been issued on the same technology.

Ravicher, a patent and trademark attorney who also specializes in open-source legal issues, said he filed the complaint because Microsoft began a campaign late last year to license some of its patents, including the patent for the company's File Allocation Table (FAT) file system. Ravicher alleges that Microsoft wants to control the licensing of the FAT system so it can discourage open-source and free software by not allowing it to be interchangeable with Microsoft software.

The issue Ravicher presented to the Patent Office, however, simply argues that US Patent 5,579,517 should never have been issued to Microsoft in November 1996 because "prior art", or evidence that someone else had previously applied for and received a patent for the idea, exists. In his letter to the patent office, Ravicher pointed out the missed prior art and asked for a review of the case.

"There's presumed to be harm on everyone if a patent is issued where it shouldn't be issued," Ravicher said. "The 517 patent is causing immeasurable injury to the public by serving as a tool to enlarge Microsoft's monopoly while also preventing competition."

Microsoft applied for the patent on 1 April1993, according to patent records. The FAT system was originally created in the mid-1970s, according to Ravicher's letter.

Microsoft spokesman Mark Martin said the company hadn't yet received a copy of the group's letter to the patent office. "We're unfamiliar with this organization and unclear why they are so interested in this one patent," he said in a statement. "Companies asked Microsoft to license our FAT specification and patents to help improve interoperability, and we have entered into a number of such licenses."

The FAT is an organizational system used to keep track of where data is stored on a storage disk so the data can be found and used by the machine. It is used in computers and other digital devices, including digital cameras and detachable storage media. The affected patent will continue to be held by Microsoft until 2013, when it will expire under US patent law.