BRUSSELS — The European Union fined Microsoft $732 million on Wednesday for failing to respect an antitrust settlement with regulators. But in a highly unusual mea culpa, the European Union’s top antitrust regulator said that his department bore some of the responsibility for Microsoft’s failure to respect a settlement that caused the fine.

Joaquín Almunia, the European Union competition commissioner, said the bloc had been “naïve” 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.

Mr. 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,” Mr. Almunia said. “Of course such decisions require strict compliance” and the “failure to comply is a very serious infringement that must be sanctioned accordingly.”