Just two weeks ago Steven Sinofsky, responsible for the overhaul of Microsoft's Windows operating system, was in front of the world's journalists brandishing a wheeled Surface tablets to show the device was so tough it could withstand being used as a skateboard.

Today he seems like just another casualty of that radical overhaul after seemingly being pushed out of the company by chief executive Steve Ballmer.

His departure marks the end of a 23-year career at Microsoft where he served as head of its Windows unit and was widely tipped to be the next chief executive of the company.

The news came out of the blue at a crucial time for the company which has thrown all its energy behind efforts to conquer the smartphone and tablet market with its new Windows 8 and Windows Phone 8 operating systems.

Neither Sinofsky, nor Ballmer has given an explanation for his departure, except brief notes that the pair left to staff at the company.

"I have always promised myself when the right time came for me to change course, I would be brief, unlike one of my infamous short blog posts, and strive to be less memorable than the products and teams with which I have been proudly and humbly associated," Sinofsky said in a letter to Microsoft workers.

"Some might notice a bit of chatter speculating about this decision or timing," he added. "I can assure you that none could be true as this was a personal and private choice that in no way reflects any speculation or theories one might read-about me, opportunity, the company or its leadership."

Ballmer's follow-up memo to staff was equally to the point and revealed little about the reason for Sinofsky's departure.

"As we enter this new era, and with the successful launch of Windows 8 and Surface behind us, Steven Sinofsky has decided to leave the company," said Ballmer. "Steven joined Microsoft in 1989 as a software development engineer and has contributed to the company in many ways from his work as a technical advisor to Bill Gates, to leading the evolution of the Microsoft Office business, to his direction and successful leadership of Windows and Windows Live as well as Surface. I am grateful for the work that Steven has delivered in his time at our company."

However, media speculation abounds that Sinofsky abrasive style and his desire to be chief executive ultimately led to his downfall.

"Sinofsky felt that he deserved to be Microsoft's next CEO, and wanted to be designated as Steve Ballmer's successor after Windows 8 shipped," said one source close to Microsoft, speaking to Business Insider.

"He threatened to quit if he didn't get the nod. Ballmer, who has previously indicated he plans to keep running Microsoft until he retires in 2017 or 2018, called his bluff."

Sinofsky had been hugely influential at Microsoft, overhauling the Office division before going over to manage the release of Windows 7 in 2009 - the most successful Windows product ever.

Sinofsky's resignation leaves Ballmer, who has been at Microsoft for 13 years, in an unassailable position in the company. In just four years of taking over of the company from Bill Gates, Ballmer has now replaced all the leaders of Microsoft's five main operating units.

Ballmer said that Julie Larson-Green, who has been with the company since 1993 and oversaw program management, user interface design and research for Windows 7 and 8, will take over from Sinofsky.