In what could become a major test case for the most widely used open source software license, a group that represents open source developers has sued Verizon Communications -- claiming that the telecom giant's FiOS broadband service violates the terms of the GNU General Public License.

In court papers filed Friday, The Software Freedom Law Center maintains Verizon's use of an open source program called BusyBox in FiOS violates version 2 the GPL because Verizon has not made the product's source code available to its customers--as the license requires.

Since November 2006, "Verizon has distributed to the public copies of the firmware in the infringing product, and none of these distributions included source code to BusyBox or offers to provide such code," SFLC alleges in its complaint, which was filed in federal court in New York.

SFLC says the offending software code can be found in Verizon's Actiontech MI424WR wireless router, a piece of equipment that's supplied to FiOS subscribers. The group filed the lawsuit on behalf of BusyBox developers Erik Andersen and Rob Landley.

BusyBox is a set of open source software tools that optimize programs to run on devices with limited resources. SFLC has previously sued three smaller manufacturers for allegedly using BusyBox in violation of the GPL. One case was settled while the other two remain pending.

The case against Verizon, however, marks the first time SFLC has sought to defend the GPL in court against a multibillion dollar industry giant. As such, it could provide a key test of the legal validity of the GPL -- a license that's used by thousands of open source developers. Some companies, including Microsoft, have said they believe parts of the GPL are not legally enforceable.

An increasing number of technology manufacturers are using open source code in their products, so a court ruling on the extent to which those manufacturers must rigidly comply with the terms of the GPL could have a significant impact on their operations.

The SFLC's suit against Verizon focuses on an aspect of the GPL that many commercial open source users find troubling -- the insistence that so-called 'downstream' end users of the licensed code--that is, customers--be given the right to see and modify the source code embedded in their products.

Some tech manufacturers, including TiVo, have claimed that the provision opens the door for users to defeat important security and copyright safeguards they've built into their products.

Open source advocates maintain that placing limits on the distribution of open source code violates their community's principals, and that companies that don't wish to use open source products are free to use commercial software.

In this latest case, SFLC is asking the court to prohibit Verizon's continued use of BusyBox and is seeking unspecified damages on behalf of Andersen and Landley.