The European Union's executive body, the Commission, is to open an investigation into whether Microsoft has kept the competition commitments it made in 2009, warning that penalties for non-compliance could be severe.
EU Competition Commissioner Joaquin Almunia said the indications are that Microsoft has failed to provide customers with a screen from which they could choose different internet browsers instead of seeing only Microsoft's product, Internet Explorer.
Mr Almunia said it appears that the choice screen, promised by Microsoft as the result of a previous anti-trust case, has not been provided since February 2011, meaning 28 million customers may not have seen it.
He said that Microsoft does not appear to be contesting the facts.
Mr Almunia added: "If infringements are confirmed, there will be sanctions."