Europe's competition commissioner Neelie Kroes delivered a thinly veiled warning to Microsoft on Tuesday, over the software giant's behind-the-scenes attempts to secure industry-wide standard status for its Office Open XML document format.

She also appeared to endorse open software standards in a speech given during a conference on standardisation hosted by the IT industry group Open Forum Europe.

She didn't mention Microsoft by name once in her speech, but the 25 minute-long delivery was littered with references to the software giant's former practice of withholding interoperability information about its near ubiquitous software products and its year-long campaign to secure ISO (International Organization for Standardization) standard status for OOXML.

"Choosing open standards is a very smart business decision indeed," she told attendees.

Standards are "the foundation of interoperability," she said, acknowledging that they can be both proprietary and non-proprietary.

After pointing out the important role proprietary standards played in developing Europe's second and third-generation mobile phone technologies, she went on to warn that standards developed and coveted by one market player only can be "problematic, having none of the safeguards of disclosure that standards bodies typically require."

In a jab at the ISO decision to grant OOXML standard status, Kroes said "I fail to see the interest of consumers in including proprietary technology in standards when there are no clear and demonstrable benefits over non-proprietary alternatives."

"If voting in the standard-setting context is influenced less by the technical merits of the technology, but rather by side agreements, inducements, package deals, reciprocal agreements, or commercial pressure then these risk falling foul of the competition rules," Kroes said.

Other speakers at the conference were more direct in their criticism of Microsoft's behaviour and ISO's decision to approve OOXML.

Christian Ude, the mayor of Munich, a city in southern Germany that has embraced open-source software throughout its operations, said that by granting OOXML standard status "it would appear this doesn't promote competition, it might even obstruct it."

One danger for public organisations from the creation of a second document format standard to sit alongside ODF (OpenDocument Format), which is already an international ISO standard, is that "scarce IT resources are needed to support the second standard," Ude said.

He added that the presence of a second document format standard will also cause "delays, and make the work of IT staff more complicated and expensive."

Microsoft declined to comment on Kroes' speech.

Microsoft was found guilty of abusing the dominant position of its Windows operating system by the European Commission in 2004. It was fined heavily and ordered to reveal important interoperability information about the OS to allow rivals to compete with it on fair terms.

At the beginning of this year, the Commission opened a fresh antitrust case against the software giant, focussing mainly on its Office software products.