BRUSSELS – In a highly unusual mea culpa, the European Union's top antitrust regulator said Wednesday that his department bore some of the responsibility for Microsoft's failure to respect a settlement that landed the company a $732 million fine.

Joaquin Almunia, the EU competition commissioner, said the union had been naive to put Microsoft in charge of monitoring its adherence to the deal it agreed to in 2009, when his predecessor let the company escape a fine in exchange for offering users of its Windows software a wider choice of Internet browsers.

But Almunia insisted that the enforcement of settlements could be sufficiently strengthened to ensure that companies abide by their pledges, and he signaled that he would not retreat from his goal to use such deals to avoid lengthy legal battles with major companies in swiftly evolving technology markets.

Settlements "allow for rapid solutions to competition problems," Almunia said. "Of course such decisions require strict compliance" and the "failure to comply is a very serious infringement that must be sanctioned accordingly."

Microsoft agreed to alter Windows for five years to give users of newly purchased computers in Europe a ballot screen that would allow them to easily download other browsers from the Internet and to turn off Microsoft's own browser, Internet Explorer.

Microsoft told the commission at the end of 2011 that it had been abiding by the deal.

"We trusted the reports about the compliance," Almunia said Wednesday.

In fact, the company had failed to include the ballot system in certain products starting in May 2011, affecting more than 15 million European users. The lapse came to light in July 2012, after rival companies reported its absence.

"We take full responsibility for the technical error that caused this problem and have apologized," Microsoft said Wednesday. "We have taken steps to strengthen our software development and other processes to help avoid this mistake – or anything similar – in the future."

A Microsoft spokesman declined to comment on whether the company would appeal the fine announced Wednesday , but it seemed unlikely, as the company prefers to focus on its rivalry with Google. Almunia said there had been no indication Microsoft intentionally broke the settlement agreement.

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