The European Commission has opened a new front in its battle with Microsoft because of suspicions that the software giant's licensing policy towards hardware manufacturers unfairly restricts competition.

It has sent 20 manufacturers formal inquiries concerning their licences with Microsoft.

The Commission is worried that the terms of the deals with companies such as IBM, Hitachi and Toshiba could impair competition by restricting the hardware groups' ability to enforce software patents of their own.

The Commission said: "We confirm that a series of letters were sent to equipment manufacturers in which we requested information on agreements they have entered into with Microsoft."