Millions of PC users will also be able to upgrade for free as Microsoft is making the update available at no extra cost to those who are already using Windows 7 or 8, though this offer will expire next year.

The firm's chief Satya Nadella has said Microsoft wants to move from "people needing Windows, to choosing Windows, to loving Windows".

Windows users who have registered their interest will be notified once Windows 10 becomes available to them, with the roll out beginning in the early hours of Wednesday morning, and set to be staggered over the coming days.

Among those who downloaded the system at the earliest opportunity was Ian Fry, of Eastbourne, who tweeted: "Currently in the process of installing 'Windows 10' on my main computer at home. Cheeky early install."

And Matt Harwood, of Hitchin, Hertfordshire, tweeted: "Installing Windows10, it seems to be taking longer than Windows7".

Those who are not eligible for the free update will have to pay £99 for the Home version of Windows 10, which will go on sale at the end of the month.

Microsoft says there are currently 1.5 billion Windows users worldwide, but the software has come under increasing pressure in recent years from Apple's OS X, which runs on their range of Macintosh desk and laptops, and has seen sales steadily climb while the rest of the market - including Windows - has struggled in the age of mobile devices.

Having also demonstrated how holographic computing can work with Windows 10 via the firm's HoloLens augmented reality headset, Microsoft is hoping to move back to the cutting edge of the technology market with its new release.