Nokia bought the Symbian software in 2008
The group behind the world's most popular smartphone operating system - Symbian - is giving away "billions of dollars" worth of code for free.
The Symbian Foundation's decision to make its code open source means that any organisation or individual can now use and modify it "for any purpose".
Symbian has shipped in more than 330m mobile phones, the foundation says.
It believes the move will attract new developers to work on the system and help speed up the pace of improvements.
"This is the largest open source migration effort ever," Lee Williams of the Symbian Foundation told BBC News.
"It will increase rate of evolution and increase the rate of innovation of the platform."
Ian Fogg, principal analyst at Forrester research, said the move was about Symbian "transitioning from one business model to another" as well as trying to gain "momentum and mindshare" for software that had been overshadowed by the release of Apple's iPhone and Google Android operating system.
Finnish mobile phone giant Nokia bought the software in 2008 and helped establish the non-profit Symbian Foundation to oversee its development.
The foundation includes Nokia, AT&T, LG, Motorola, NTT Docomo, Samsung, Sony Ericsson, STMicroelectronics, Texas Instruments and Vodafone. It's useful for them to say Symbian is now open - Google has done very well out of that
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The group has now released what it calls the Symbian platform as open source code. This platform unites different elements of the Symbian operating system as well as components - in particular, user interfaces - developed by individual members.
It can be downloaded from the foundation's website.
Mr Williams said that one of the motivations for the move was to speed up the rate at which the 10-year-old platform evolved.
"When we chatted to companies who develop third party applications, we found people would spend up to nine months just trying to navigate the intellectual property," he said.
"That was really hindering the rate of progress."
Opening up the platform would also improve security, he added.
Symbian development is currently dominated by Nokia, but the foundation hoped to reduce the firm's input to "no more than 50%" by the middle of 2011, said Mr Williams.
"We will see a dramatic shift in terms of who is contributing to the platform."
However, said Mr Williams, the foundation would monitor phones using the platform to ensure that they met with minimum standards.
Despite being the world's most popular smart phone operating system, Symbian has been losing the publicity battle, with Google's Android operating system and Apple's iPhone dominating recent headlines.
"Symbian desperately needs to regain mindshare at the moment," said Mr Fogg.
"It's useful for them to say Symbian is now open - Google has done very well out of that."
He also said that the software "may not be as open and free as an outsider might think".
"Almost all of the open source operating systems on mobile phones - Nokia's Maemo, Google's Android - typically have propriety software in them."
For example, Android incorporates Google's e-mail system Gmail.
But Mr Williams denied the move to open source was a marketing move.
"The ideas we are executing ideas came 12-18 months before Android and before the launch of the original iPhone," Mr Williams told BBC News.