SEATTLE - The person most widely tipped to be the next chief executive of Microsoft Corp resigned from the world's largest software maker barely two weeks after launching the flagship Windows 8, as chief executive Steve Ballmer moved to tighten his grip on the company.

The exit of 23-year company veteran Steven Sinofsky, head of Microsoft's Windows unit, is the latest - and most prominent - in a line of high-profile departures from the Redmond, Washington-based company, which is struggling to keep pace with Apple and Google in mobile computing.

Thew move was unexpected and neither Microsoft nor Sinofsky gave an explanation, although a senior executive at the company, who asked not to be named, said the decision was "mutual" and said he was not expecting Sinofsky to take a job at another company soon.

"This is shocking news. This is very surprising," Brendan Barnicle, an analyst at Pacific Crest Securities, said.

"Like a lot of people, I thought Sinofsky was in line to potentially be Ballmer's successor."

Ballmer told employees in a memo on Monday simply that: "Steven Sinofsky has decided to leave the company."

In a later media statement, he added that it was "imperative that we continue to drive alignment across all Microsoft teams, and have more integrated and rapid development cycles for our offerings".

That could be interpreted as disappointment inSinofsky, 47, known as an uncompromising leader who wielded immense power as head of the Windows unit, the traditional centre of Microsoft's business, but was not known for working well with other executives.

Ballmer, 56, shows no sign of leaving after almost 13 years in the job despite a flat stock price for the last decade.

He has now replaced all the leaders of Microsoft's five main operating units in the past four years.