In another blow to the supremacy of Microsoft's Office franchise, Belgium on Friday became the second governmental body to approve the use of the OpenDocument format as a way to exchange government documents.

By September 2007, all Belgian federal agencies must use software that can read reports, spreadsheets, presentations and other types of data files saved in OpenDocument (ODF), a free XML file format certified as a standard by the International Standards Organization (ISO) last month.

"If the impact analysis shows no adverse impact, ODF might even become the standard exchange format in September 2008," according to information posted on the Web site of the Belgian Government Interoperability Framework. Belgium joins the state of Massachusetts in a symbolic break from Microsoft. Massachusetts plans to make ODF its standard for all official government documents by Jan. 1.

Though it is a small country with just 10 million citizens, Belgium's embrace of ODF could have big ripple effects. Its capital, Brussels, is the headquarters of the European Union, making it an important political center in Europe.

The Danish and Norwegian governments are also considering moving to OpenDocument, according to Jason Faulkner, a member of the OpenDocument Fellowship, an ODF advocacy group.

"Given the current favorable attitude of the European Community to open standards, and, in particular, to the way that European governments and initiatives are defining open standards, it is not surprising that a growing number of European countries are moving to adopt ODF, which is conducive to not only proprietary but open-source implementations as well," said Andy Updegrove, an open-source advocate and Boston-based lawyer.

ODF is the default file format in OpenOffice, StarOffice and an increasing number of Web-based word processing and spreadsheet applications. Though only utilized by a small percentage of users worldwide today, ODF is supported by some of the largest technology vendors, including IBM, Sun Microsystems and Novell. They argue that the format's open, interoperable nature makes it suitable for groups concerned about long-term archiving of files.

ODF's chief opponent is Microsoft, whose market-leading Office suite is used by more than 400 million people worldwide, according to the vendor. By default, Office applications such as Microsoft Word, Excel and PowerPoint save files in proprietary file formats owned by Microsoft, such as .doc, .xls and .ppt, respectively -- although users can also choose open formats such as Text (.txt) and Rich Text Format (.rtf).

Some experts say that users' need to remain compatible with Office formats has been key to maintaining Microsoft's market dominance. Microsoft has argued that ODF stifles its own innovation and limits customer choice. It is developing a competing format, OpenXML, that will debut in its forthcoming Office 2007. Microsoft has garnered the support of other vendors for OpenXML and is applying to ECMA International, a rival standards body to ISO, for certification as an open standard.

"We understand the Belgian government's desire to support open-standard document formats and understand that other formats can qualify over time," said Alan Yates, Microsoft's general manager for information worker business strategy. "We believe that OpenXML formats will meet the government's criteria shortly."

In Massachusetts, the controversy that followed last year's decision to adopt ODF led to the resignation of then state CIO Peter Quinn. His successor, Louis Gutierrez, last month put out a search for third-party plug-in software that would enable Office users to easily read and save files in the OpenDocument format.

Massachusetts is now testing those plug-ins, according to Gary Edwards, a software developer who has created plug-ins for Word and Excel and is working on one for PowerPoint. Edwards said his plug-in allows Office users save files in the OpenDocument format without losing formatting information and can open those files up to 25 percent faster than equivalent Office files.

Such plug-ins could enable Massachusetts to continue using Office in the short run or indefinitely while fulfilling its requirement to move to ODF, Gutierrez said. It is unclear whether such plug-ins, combined with Microsoft Office, would meet the mandate from the Belgian government.