The European Union fined Microsoft Corp 561 million euros (US$731 million) yesterday for failing to offer consumers a choice of web browser, a charge that will act as a warning to other technology firms involved in antitrust disputes with the EU.

It said the US company had broken a legally binding commitment made in 2009 to ensure consumers had a choice of browser, rather than defaulting to Microsoft's Internet Explorer.

An EU investigation found that Microsoft had failed to honor that obligation in software issued between May 2011 and July 2012, meaning that 15 million users were never made aware that they could choose.

"Legally binding commitments reached in antitrust decisions play a very important role in our enforcement policy," said Joaquin Almunia, the EU's competition commissioner.

"A failure to comply is a very serious infringement that must be sanctioned accordingly."

Yesterday's fine is the first time the European Commission, the EU's antitrust authority, has fined a company for non-compliance with agreed commitments. It could have charged Microsoft up to 10 percent of its global turnover, or as much as US$7.9 billion.

In that respect, the fine is relatively light, but still marks a firm sanction by the EU and will not go unnoticed by the likes of Google, which is involved in a dispute with the commission over how it ranks search engine results.

Google is under pressure to offer concessions to prevent the antitrust authority moving to the next stage in the case, which could involve fines. Other major technology firms are also in the commission's crosshairs in other cases.

Microsoft has a long and bitter relationship with the EU's powerful antitrust authority, which has now issued fines totaling 2.16 billion euros against the US firm.

In 2004, the commission found that Microsoft had abused its dominant market position in relation to the tying of Windows Media Player to the Windows software package and imposed fines.

Then in 2009, in order to resolve other competition concerns, Microsoft agreed to offer users a browser choice screen.