Microsoft has been formally accused of failing to comply with a European order to give customers a choice among internet browsers.
The European Commission had told the software giant it must offer users a choice among browsers to ensure the company was not using its dominant market position to foist its internet Explorer on them.
Microsoft created a “choice screen” but in July conceded that a technical error meant that millions of users in the EU may not have seen it between February 2009 and July 2012.
The Commission formalised its complaint, although the investigation continues.
Microsoft could face a fine of up to 10% of its annual revenue if found in breach of anti-competition law.