Microsoft appears to be edging closer to releasing Windows 10 Mobile for Lumia smartphones after making an "Upgrade Advisor" app available to internal beta testers. The app provides more information on Microsoft's latest upgrade, and helps users prepare their device for its release.

"The Windows 10 Mobile Upgrade Advisor app will tell you if your Windows 8.1 phone is eligible for upgrade to Windows 10 Mobile," reads the official blurb. "It can also help free up space on your phone so you're ready for the upgrade."

When you open the app for the first time, it will check to see whether Windows 10 is ready to download to your device, or whether any additional updates are required to prepare your device for Windows 10. If you don't have enough space available, it will suggest content you can remove.

The app can also move files to OneDrive for you, so that you can delete the local copies to free up storage space.

This app was spotted on the Windows Store, but right now, it's only available to internal beta testers, according to Neowin. Providing internal testing goes well, however, we expect Upgrade Advisor to be available soon, followed by Windows 10 Mobile.

Microsoft originally promised Windows 10 Mobile would be available to existing Windows Phone users with eligible devices in December, put it pushed the release back to January late last year. The company is still yet to provide a definitive release date.