Microsoft is to learn the hard way that “a deal is a deal,” at least in the eyes of the European Union, by being forced to swallow a massive fine for breaching earlier promises made with the 27-nation bloc.

The software giant has been fined 561 million euros ($731 million) by European authorities for falling foul of previous antitrust settlement conditions.

The software giant breached a settlement that it signed with the European Commission in 2009, which mandated that it display a “browser choice” screen on all existing and new PCs in the region.

Europe’s antitrust and competition chief, Joaquin Almunia, stressed the importance of maintaining the legally binding commitments.

“In 2009, we closed our investigation about a suspected abuse of dominant position by Microsoft due to the tying of Internet Explorer to Windows by accepting commitments offered by the company.”

He added: “Legally binding commitments reached in antitrust decisions play a very important role in our enforcement policy because they allow for rapid solutions to competition problems. Of course, such decisions require strict compliance. A failure to comply is a very serious infringement that must be sanctioned accordingly.”

A Microsoft spokesperson in Brussels said in an e-mail to ZDNet that the company takes “full responsibility for the technical error.” He added:

“We provided the Commission with a complete and candid assessment of the situation, and we have taken steps to strengthen our software development and other processes to help avoid this mistake — or anything similar — in the future.”