Andy Rubin, head of Google's Android mobile operating system, has stepped down.

Rubin had led the Android operation since its inception, first as an independent business and then at Google after its sale to the search giant in 2005.

He leaves Android in fairly rude health: its position as the main alternative to Apple's iOS is firmly established, and by some measures it is now the biggest mobile OS in the world. It has been activated on more than 750 million devices, made by 60 manufacturers and more than 25 billion apps have been downloaded from Google Play.

Rubin will remain at Google to work on new projects, which CEO Larry Page described as "moonshots". Some have speculated that may include Google Glass, the company's new wearable computing initiative, but that is unconfirmed.

"Andy's decided it's time to hand over the reins and start a new chapter at Google. Andy, more moonshots please!" Page said in a blogpost.

Google's current head of Chrome and Apps, Sundar Pichai, will take over Android in addition to his current role.

That choice hints at a closer integration of Chrome and Android, and possibly Chrome OS, Google's desktop operating system.

It has been rumoured that Chrome OS will soon be updated to run Android apps - though that remains speculation.

Page continued in his blog post:

Sundar has a talent for creating products that are technically excellent yet easy to use--and he loves a big bet. Take Chrome, for example. In 2008, people asked whether the world really needed another browser. Today Chrome has hundreds of millions of happy users and is growing fast thanks to its speed, simplicity and security. So while Andy's a really hard act to follow, I know Sundar will do a tremendous job doubling down on Android as we work to push the ecosystem forward.