EU antitrust regulators are investigating whether Microsoft blocks computer makers from installing web browsers other than its Internet Explorer (IE) on its upcoming Windows 8 operating system.

The issue is part of an investigation into Microsoft's failure to offer users a choice of rival web browsers in its Windows 7 OS, which it had agreed with regulators three years ago.

The EU watchdog is also looking into allegations that Microsoft does not provide access to complete interfaces (APIs) for non-default browsers in Windows 8.

The investigation will also focus on charges that Microsoft allows only its own Internet Explorer browser to be installed on devices running Windows 8 on Windows RT tablets with British chipmaker ARM's chips.

Windows 8 is due out October 26.