Microsoft has been fined €561m (£484m) in Europe for failing to promote a range of web browsers, rather than just Internet Explorer.
The software giant was forced to introduce a browser choice screen back in 2010 as part of an antitrust settlement with the EU.
However, Microsoft dropped the feature in a Windows 7 update in February 2011.
Microsoft has now reintroduced the browser choice facility after being alerted to the fact it was missing.
Microsoft said the omission had been the result of a “technical error”.
But competition commissioner Joaquin Almunia said the action was unprecedented, adding he wanted to deter any company from the “temptation” of reneging on such a promise.
In theory the watchdog could have fined the firm 10% of its global annual revenue, which would have totalled $7.4bn based on its 2012 report.
“We take full responsibility for the technical error that caused this problem and have apologised for it,” a spokesman for Microsoft said following the announcement.
“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.”
At a press conference in Brussels, Almunia said Microsoft’s lack of compliance represented a “serious breach” and was the first time a firm had failed to meet such a commitment.
He explained that he preferred negotiated settlements, rather than extended legal battles, when tackling competition complaints in the fast moving IT sector.
But he added that Microsoft’s willingness to co-operate with the EU’s subsequent investigation had acted as a mitigating factor when determining the level of the fine.
“I hope this will make companies think twice before they ever thinking of breaching their international obligations,” said the commissioner.
Microsoft’s chief executive Steve Ballmer and the former head of its Windows division Steven Sinofsky have already had their most recent bonuses docked, in part because of the browser affair.
Read the official statement from the EU competition commission here.