An unstable internal build of Windows 10 was accidentally released to people signed up to test early releases of the OS under the Windows Insider program.

Microsoft broke a number of Windows 10 smartphones yesterday by pushing out a buggy release of the OS.

The internal build of Windows 10 was accidentally sent to people signed up to test early releases of the OS under the Windows Insider program.

However the version released yesterday, Build 16212, was not ready for public use, and Dona Sarkar, head of the Windows Insider Program at Microsoft, put out a warning not to install the release.

#WindowsInsiders: pls do not install any builds being offered til you hear from us with a blog post. If you have installed, reset with WDRT. -- Dona Sarkar (@donasarkar) June 1, 2017

Any mobile users who installed the buggy build found their device stuck in an endless reboot loop.

Microsoft has since blocked the release from being pushed to any more Insiders, and Sarkar said only a limited group among the millions of Windows Insiders were affected.

"Our analysis shows only a small portion of folks got these builds," she said.

Mobile users who installed the unstable release on their phones have to use the Windows Device Recovery Tool and reflash their device to get it working again. If they so wish, they will need to rejoin the Windows Insider Program again after flashing their device.

SEE: Windows 10 upgrade destroyed data and damaged PCs, claims multi-million dollar lawsuit

If a mobile device has downloaded the build but not installed it, users will see an 'Install' or 'Restart now' button under Phone update. In this instance, users need to reset the device via Settings -> System -> About. Microsoft advises first backing up the device via Settings -> Update & security -> Backup.

The pre-release version of Windows 10 was also pushed out to Windows Insiders on PCs and while not breaking devices may be more unstable than typical Insider builds of Windows 10, according to Sarkar. PC users can roll-back to the previous build via Settings -> Update & security -> Recovery.

The release doesn't affect Windows 10 users beyond those signed up to the Insider Program. Last year there were numerous complaints about bugs introduced by the Anniversary Update, however this year's Creators Update seems to have caused fewer issues.

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