Microsoft has won a year-long quest to make its Office Open XML document format an ISO-recognised international standard.

But when the Commission, Europe's top antitrust authority, opened a probe into Microsoft's business practices in January, it said part of the investigation would examine whether OOXML, as the format is known, is "sufficiently interoperable with competitors' products."

A month later the Commission sent a confidential request for information to all the national divisions of the ISO (International Organisation for Standardisation) in Europe, asking for information about the ongoing process of assessing OOXML.

"In your opinion, have there been any irregularities or attempts to influence the debate or vote on the ECMA 376 proposal as regards your organisation? If so please provide details and any relevant facts," the Commission wrote in the letter, a copy of which was obtained by IDG News Service. ECMA 376 is the title under which Microsoft submitted OOXML for consideration by the ISO.

The request for information, known as an Article 18 letter, is a formal procedure carried out by the Commission's antitrust officials, designed to gather evidence of antitrust abuse.

It was used in the first Microsoft investigation, which concluded in 2004. The replies the Commission received that time led to fresh antitrust charges that Microsoft had been unfairly bundling its Media Player with its Windows operating system.

If national ISO bodies return evidence that Microsoft attempted to influence the votes to secure acceptance of OOXML, it would strengthen the Commission's antitrust case.

One ISO official from Norway has already spoken of "serious irregularities" in the voting process in his country. Steve Pepper, chairman of the committee advising the Norwegian branch of the ISO, wrote to the central office of the ISO in Geneva, Switzerland, on Monday, recommending that the Norwegian vote be excluded from the final tally.

Norway voted against granting OOXML ISO-standard status in the first round of votes last year. In the vote taken last weekend, it changed its mind and supported Microsoft's bid.

"You will have been notified that Norway voted to approve OOXML in this ballot. This decision does not reflect the view of the vast majority of the Norwegian committee, 80 per cent of which was against changing Norway's vote ... to yes," Pepper wrote in the letter, which he also posted in his blog.