How to fix '32bit only' guest OS issue in VirtualBox

Recently installed on one of my test machines latest version (5.2.20) of VirtualBox and found that it's not possible to install and run properly a 64bit guest OS, the only option was 32bit. The culprit was 'Hyper-V' enabled feature on Windows 8.1 x64 while Intel's VT-x was enabled in BIOS (AMD-v for AMD cpus). So, to remove this '32bit only' restriction, go to 'Control Panel > Programs and Features > Turn Windows Features on or off' and uncheck the box in front of 'Hyper-V', click OK and restart the machine. Problem solved