The European Commission has fined Microsoft 561 million euro for breaking the terms of an earlier agreement to offer users a choice of internet browser.

Microsoft thought it had put its regulatory woes behind it with the 2009 settlement in which it paid 860 million euro  and promised to give Windows users the option of choosing another browser rather than having Internet Explorer automatically installed.

But Microsoft failed to stick to the deal for some 15 million installations of Windows 7 in Europe from May 2011 until July 2012. The company acknowledged the failure and said it was a mistake.

The commission's top regulator, Joaquin Almunia, said negotiated settlements are vital for enforcement and this was the first time a company had failed to keep its word.

The penalty is a first for Brussels - no company has ever failed to keep its end of a bargain with EU authorities before.

Mr Almunia added that the large fine took into account the size and length of time the company violated the terms of its agreement, as well as the need to deter other companies from backsliding on their promises to competition authorities. He said the fine was less than it might have been because Microsoft had co-operated with the investigation.

A Microsoft spokesman said the company would respond later today. In all, the company has now paid a total of 2.2 billion euro  in fines to the commission since it began its first investigation into the company's abuse of market power linked to its dominant Windows operating systems in 1998.