In the latest challenge to Microsoft's stranglehold on the productivity software market, IBM said Monday that it will donate part of the code behind its Lotus Notes package to OpenOffice.org, a Sun Microsystems-backed group that offers open source office software as a free download.

IBM officials said the move is part of a broader effort that IBM is undertaking to support OpenOffice.org, which produces free word processing, spreadsheet, and presentation software that competes with Microsoft's pricey Office 2007 suite.

IBM's support for OpenOffice will help "accelerate the rate of innovation in the office productivity market," said Mike Rhodin, general manager of IBM's Lotus division, in a statement. Among other things, IBM plans to donate to OpenOffice.org some of the code that it uses to make its Lotus products accessible to visually impaired users.

IBM's contribution to OpenOffice.org represents a give-back of sorts. The company used OpenOffice's open source software to build some of the productivity applications embedded in Lotus Notes 8.

IBM's participation in the project could make OpenOffice a more serious threat to Microsoft. The group is competing with Redmond not just on finished products, but also on the document formats underlying them.

OpenOffice software is based on the Open Document Format, which has been certified as a standard by the International Organization for Standardization. A proposal that would have given fast-track ISO approval to Microsoft's rival product, Office Open XML, was voted down last week by ISO member countries. Microsoft has said it will continue to seek ISO approval for OOXML.

The stakes are high as an increasing number of government and enterprise software buyers are insisting that the applications they purchase conform to internationally recognized standards.

OpenOffice isn't the only challenge to Microsoft's dominance in the productivity software market. Google recently entered the fray with an array of hosted applications sold as a service under the Google Apps brand. On Monday, outsourcer Capgemini said it would add support for Google Apps Premier to its service offerings.