FRANKFURT (Reuters) - A Microsoft MSFT.O document format that may be adopted as an international standard this weekend is a ploy to lock in customers, who could lose control over their own data in a worst-case scenario, critics say.

The International Organisation for Standardisation (ISO) is balloting its members on the issue in a vote that closes on Sunday. ISO approval would encourage wider adoption of the Microsoft Open XML format by public-sector organizations.

Opponents of Open XML, which is the default file-saving format in Microsoft Office 2007, say there is no need for a rival standard to the widely used Open Document Format (ODF) that is already an international standard.

They argue its 6,000 pages of code, compared with ODF’s 860 pages, make it artificially complicated and untranslatable.

Microsoft and others point out that multiple standards are normal in the software and other industries and that competition makes for better products. Microsoft says its format has higher specifications and is more useful than ODF.

“More parallel standards makes for better standards. It’s good not to decide for a single standard too soon,” Michael Groezinger, Microsoft’s chief technology officer in Germany, told Reuters in an interview.

He declined to speculate on the outcome of the ISO vote but welcomed last week’s decision of the German Institute for Standardisation -- an ISO member -- to give Open XML a conditional “yes” vote.

At the heart of the controversy are fears that Open XML is not as open as it claims to be, raising the specter that customers using the word-processing format could become reliant on Microsoft for access to their own documents.

XML, short for Extensible Markup Language, is a standard for describing data in a way that is supposed to allow it to be shared across various systems and applications.

“The absolute nightmare scenario is that Microsoft says: ‘Update your licenses or we’ll turn off your access,’” Georg Greve, president of the Free Software Foundation Europe, told Reuters in an interview.

“Access to governmental data will completely depend on the existence of Microsoft,” said Greve, who expects Microsoft to lose the ISO ballot in a close vote.

The Free Software Foundation is a U.S.-based non-profit organisation that campaigns for computer programs that can be freely used, modified and redistributed.

Microsoft’s Groezinger denies any danger of bodies losing access to their own data. He said Microsoft had handed over control of Open XML to standards-making body Ecma, which would make it available even in the event of Microsoft’s demise.

Microsoft has also given guarantees not to pursue any patent claims against parties using, selling or distributing Open XML, although some have questioned whether those guarantees are sufficiently binding or comprehensive.

It has also collaborated with Novell NOVL.O to develop a tool to translate Open XML documents into ODF and vice versa. But critics say the tool cannot provide a complete translation due to the higher complexity of the Microsoft format.

Open XML is unnecessarily bloated, partly because it packs in unrelated features that lead users to other Microsoft applications, FSF’s Greve says.

“This is a classic vendor lock-in strategy,” he told Reuters. “It’s not that new, it’s not that ingenious but it’s quite effective.”

Given Microsoft’s leading market position, Open XML will become a de facto standard regardless of the ISO decision.

“The two standards may converge in the longer term, but all organizations should plan on them coexisting for at least the medium term,” research group Gartner said in a recent report.

“The problems associated with the need to translate between formats will continue and will diminish the value of XML.”