Windows 10 for smartphones is already being tested internally at Microsoft according to a veteran Windows leaker and former Microsoft employee.
Nazwil Najeeb claimed on Twitter that Microsoft is currently testing three new versions of Windows for phones, including version 8.15.12434, which is Windows 10 according to Najeeb.
Microsoft has revealed a few little details about the 'desktop' Windows 10, mostly relating to how its start menu will work, but we've not seen anything phone-specific to date beyond a teaser image.
However, even the tiny image seen at Windows 10's announcement in September suggests Microsoft is sticking with the Live Tiles home screen style used in the system since Windows Phone 7.
No sign of 9
Skipping Windows 9 altogether, Windows 10 will work across phones, tablets, laptops and desktops, as cemented in the system's "one product family, one platform, one store" launch tagline.
First Microsoft needs to roll-out the other two Windows Phone 8.1 builds Najeeb mentions - update 1 and update 2.
Microsoft has previously claimed there would not be a Windows Phone 8.1 update 2, suggesting the "8.10.15XXX" update Nazwil has spotted may be a minor update that the company will not make a grand song and dance about.
Windows Phone 8.1 Update 1 brings Cortana to the UK, adds support for more screen resolutions and enables folders on the start screen.
There's no real mystery to this update, though, as it has already started rolling-out to some devices including the Nokia Lumia 930.