Chief executive Satya Nadella made the announcement while briefing analysts following Tuesday’s earnings update.
“We will streamline the next version of Windows from three operating systems into one single converged operating system for screens of all sizes,” he said.
The firm also confirmed it had recently scrapped a new type of tablet.
“During the quarter, we reassessed our product roadmap and decided not to ship a new form factor that was under development,” said Amy Hood, the firm’s chief financial officer.
Leaks had indicated that the firm had originally planned to launch the Surface Mini in May – a small tablet running the Windows RT system, a version of the OS designed for ARM-based chips.
Mr Nadella did not mention Windows RT during the conference call, instead referring to the three existing versions of Windows that would be merged as “one for phone, one for tablets and PCs, [and] one for Xbox”.
One analyst speculated that Mr Nadella saw Windows RT as a dead end. The platform was launched in 2012, but still lacks several big name applications including the full version of Photoshop and many games.
“Windows RT devices haven’t sold in the way Microsoft hoped they would, certainly on the tablet side,” said Chris Green, principal technology analyst at the Davies Murphy Group consultancy.
“The developer take-up was also lower than expected.
“There has, however, been good steps forward in the power-savings options for ultra-low-power x86 chips, and they are now getting to the stage where they could be used to power smartphones running a single Windows code set.”
Mr Green noted that designing the new unified Windows system to run on x86 processors, rather than trying to make a version for ARM-based chips, would make it simpler to create.
Such a move would play to the favour of chip manufacturers Intel and AMD at the expense of Qualcomm, whose processors are used in Microsoft’s current Lumia smartphones.
However, Mr Nadella did not comment on this point during the call, and a spokeswoman for Microsoft was unable to provide more detail.
Mr Nadella did, however, discuss the benefits of merging the Windows Phone OS with the other versions of the system, noting that it would swell the number of programs available to run on the firm’s handsets as a result of having “one ecosystem”.
“One of beauties of universal Windows app is it aggregates for the first time for us all of our Windows volume,” Mr Nadella said.
“An app that runs with a mouse and keyboard on the desktop can be in the store, and you can have the same app run in the touch-first [mobile devices].
“[It] gives developers the entire volume of Windows, which is 300 plus million units as opposed to just our 4% share of mobile in the US or 10% in some countries.”
The move puts Microsoft at odds with Apple and Google, which are both pursuing separate strategies for laptop/desktop computers and mobile devices – Apple with Mac OS X and iOS, Google with Chrome and Android.
But it does bring Microsoft closer to another OS developer, Canonical, which has promoted the idea of its Ubuntu system powering both phones and desktops. Canonical previously highlighted that one benefit of this strategy was that a handset could double up as a low-power desktop PC if it was plugged into a monitor and connected to a mouse.
It also paves the way for Microsoft to introduce its voice-controlled personal assistant, Cortana, to PCs. Mr Nadella mentioned the app several times during the call.