You can add Microsoft's Office to the business divisions at the tech giant under attack by plucky upstarts like mobile app maker Infraware. But breaking Redmond’s corporate choke-hold won't be easy.

Effectively shut out of the smartphone operating system market by Apple and Google's Android, the software giant has responded by pushing hard into tablets with the costly and poorly-received Surface, and then an even more expensive buyout of Nokia's struggling handset business.

Now Microsoft's sacred cash cow - the Office suite - is facing its own mobile-driven crisis.