The firm announced that the OS upgrade would be offered free of charge for devices running Windows 8, Windows 7 and Windows Phone.

The offer, which is limited to the Windows 10's first year of release, may aid its adoption.

Windows 10 brings the same operating system to devices of all sizes, rather than having different ones for PCs/tablets, mobile phones and the firm's Xbox games console.

Microsoft has revealed that Windows 10 will bring its voice-controlled assistant Cortana to PCs which was previously limited to Windows Phone handsets.

The company says that the software could be told to many actions such as locate photos taken during a specific month, dictate and send emails and show where their car is parked.

It also unveiled a headset - the HoloLens headset that it said would one day project the operating system over views of the real world.

Microsoft says its helmet will blend 3D transparent graphics with the real world.