The approval clears the way for a vote by ISO member organisations, which ultimately determines OpenXML's status as an open format.
The voting process will take five months. ISO procedures allow for additional changes to be made if it is voted down.
Approval by the 157 member countries of the ISO turns the OpenXML standard into an official open standard. The format is backed by Microsoft, which is hoping to fast-track the approval process.
OpenXML describes how documents in applications such as Microsoft Word and Excel are formatted. OpenXML competes with the Open Document Format, which is backed by companies including IBM, Sun Microsystems and Adobe.
The advance comes despite objections from 20 of the 30 countries that reviewed the format during the initial review stage. OpenXML opponents initially speculated that the objections would delay the approval process.
But an ISO-approved poll by Computerworld indicated that only six countries opposed the fast-tracking of OpenXML, with the remaining filing more minor objections.
A speedy approval of OpenXML is important to Microsoft now that numerous governments and educational institutions are mandating the use of an open document format.
This ensures that documents can be accessed and edited in the future, even after today's text editing software has been mothballed.
Until the standard is approved, such institutions will be forced to replace Microsoft software with alternative suites such as OpenOffice, or use special translator software which converts documents between the ODF and OpenXML formats.
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