Nokia has announced its intention to buy the remaining 52 per cent of Symbian shares that it does not already own.
The mobile giant will pay around €264m in cash to buy the shares from Sony Ericsson, Telefonaktiebolaget, Panasonic and Siemens.
Samsung has not yet agreed to give up its stake, but is expected to do so soon.
The acquisition will enable Nokia, Sony Ericsson, Motorola and NTT DoCoMo to combine the Symbian OS, S60, UIQ and MOAP(S) to create a single open mobile software platform.
The partners will form the Symbian Foundation to drive the new platform, together with AT&T, LG, Samsung, STMicroelectronics, Texas Instruments and Vodafone.
"This is a significant milestone in our software strategy," said Olli-Pekka Kallasvuo, chief executive of Nokia.
"Symbian is already the leading open platform for mobile devices. Through this acquisition and the establishment of the Symbian Foundation, it will undisputedly be the most attractive platform for mobile innovation.
"This will drive the development of new and compelling web-enabled applications to delight a new generation of consumers."
By making Symbian an open source platform, Nokia is directly squaring up to Google's upcoming Android mobile OS, which recently reported delays.
Kallasvuo claims to have seen strong demand from mobile developers for an open and widely used platform to help drive innovation and unified application development.
Currently, over four million developers are engaged in producing applications for Symbian devices.
Around 60 per cent of mobile devices run on Symbian OS, and more than 200 million Symbian OS-based phones have been shipped on over 235 models from eight vendors.
"Ten years ago, Symbian was established by far sighted players to offer an advanced open operating system and software skills to the whole mobile industry, " said Nigel Clifford, chief executive of Symbian.
"Our vision is to become the most widely used software platform on the planet and today Symbian OS leads its market by any measure."
Nokia expects the acquisition to be completed before the end of the year, at which point all Symbian employees will join Nokia.
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