The penalty is the latest episode in over a decade of wranglings between the EU and Microsoft, which has already seen the commission fine Microsoft €1.6 billion for failing to provide rivals with information at fair prices and for tying its media player to its operating system.

BRUSSELS — Microsoft Corp. has been hit with a €561 million ($732.2 million) fine by European Union regulators after it broke its promise to offer millions of users of its Windows system a choice of rival Web browsers.

The European Commission said it was imposing the fine after the U.S. software giant became the first company to break a voluntary agreement with regulators, which would have allowed at least 15 million consumers to pick alternatives to its Internet Explorer browser. ...