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Ray Ozzie, chief software architect of Microsoft and proponent of cloud computing, has resigned unexpectedly.

Mr Ozzie was a top member of the company's management, having taken over the software role from Bill Gates.

Chief executive Steve Ballmer announced his colleague's departure in an email to company staff.

He said Mr Ozzie would remain with Microsoft during a transitional period, and that the company was not looking for anyone to replace him.

Microsoft shares dropped 2.2% in after-hours trading on the news.

"With our progress in services and the cloud now full speed ahead in all aspects of our business, Ray and I are announcing today Ray's intention to step down from his role as chief software architect," .

He added that Mr Ozzie would focus on "the broader area of entertainment, where Microsoft has many ongoing investments".

Nonetheless, his resignation may cast some doubt over the technological direction Microsoft will take next.

Mr Ozzie's decision to step down also follows a number of other senior departures at the company.

Business head Stephen Elop left in September to head up Nokia, while entertainment and devices head Robbie Bach is also planning to leave.

Mr Ozzie joined the firm in 2005 as chief technology officer after his own company was bought out by Microsoft.

A year after his arrival, he successfully pushed the software leviathan towards tackling the challenge of the internet, by adopting "cloud computing" technology.

Microsoft traditionally focused on desktop computers, but Mr Ozzie convinced management that in the future, processing power and functionality would be provided remotely via the web.

Previously, he also designed the Lotus Notes system, which allows groups to share documents and emails.